This is bordering 'what to write', but I'll add some things to @Galastel's very nice answer, because this could probably help other writers struggeling with similar questions.
Almost any story has more than one character/person that we follow, either directly or indirectly.
You obviously have your MC, but the side characters in your story can be used to explore sides of your MC and the story:
You've got events in your story that your MC doesn't remember - Other people must remember them, unless she's been completely isolated for over 2 years, which seems unlikely, yet is up to you.
You can use side characters to interact with her in a way that makes her realize that which she doesn't know/remember.
If you're aiming for a somewhat creepy story, this'll be a neat trick: If she trusts the person she's talking to, AND that person suddenly starts talking about her recent past, some part of her has to believe that person, even if at first she thinks it's a 'prank'.
3. Physiology / Psychology
I don't know your age, and it doesn't necessarily matter, but:
People age, always (unless you're in some sci-fi/fantasy genre). Some periods of our lives have a stronger impact on how old we physically appear and behave, but especially stress, be it physical or mental will tear on us tremendously.
If your MC has gone through pregnancy (and birth?) she will look different, unless she is magically immune to the physical and mental effects connected to it. Some things you could consider connected to the pregnancy:
As human beings, we're biologically engineered to, at some point, feel mentally and physically ready for offspring (have a child). This is also a part of what most people consider 'maturity'.
This will show itself in various ways; We will look at the world differently - notice babies, perhaps be attracted to different things from what we used to, we may change our appearence to fit what we are suddenly attracted to.
Also, aging in itself alters our appearence. At some 'stages' in our lives, our age can be hard to tell - and a lot of people never 'look their age' - but if your MC has missed 2 years of her mid-twenties, I strongly believe she would notice herself looking different. Especially if she has been through pregnancy and events connected to that alone.
I'm 28 (not a tip), and a lot of these days, I feel as if I look 'tired'.
Granted, I probably never really get enough sleep - Who does in this day and age? - BUT; This could simply be my mind's way of coping with the fact that I am actually starting to look older. I am not the young man I became at the end of my teen years.
Have your MC notice small things about her body - Perhaps look up 'aging signs mid to end twenties' to find things she might notice.
Connect that with the people in her life: Does she have siblings or same-aged friends? Does she have a job with colleagues? They must look older too!
If she has lived her life normally, but simply doesn't remeber the last 2 years (I hope you've got a great explanation, even if you keep it to yourself), most of the clues or - to her, discrepancies - will be in her head through the things she considers, notices and realizes.
I don't randomly walk around asking friends or family if they remember 'this and that', if they are obvious things to remember - like being pregnant. However, people like to talk about the fun/odd/lucky/out-of-the-ordinary things that happen that are connected to the obvious things.
A friend might say: "I can't believe your belly i so flat again already. Do you even have stretch marks?"
This, however, would indicate that her pregnancy is something that 'they' can talk about, meaning it is probably not a traumatic event. If she lost her child, you probably wouldn't 'compliment her body of getting over the pregnancy'.
A nice 'trick' you could use is to have her pregnancy be somewhat taboo, meaning that the people that know her, don't talk about it - making it more difficult for her to find out.
So, to sum up:
The 1, 2 and 3 should come together in a web of experiences that makes your MC notice things that are slightly or completely 'off'.
Have your MC meet people that know her.
Have her confused when someone is talking about something in a manner, where it seems she should know what they are talking about - that's usually how people talk to someone they know.
Have her notice small changes about herself and the people she interacts with.
What she finds out is up to you, but using side characters is a powerful tool for showing that something is in a certain way.
It has been explained that the MC is not the only one suffering from memory loss. In that case, the MC and the side chraracters could instead notice various changes in eachother first, and then themselves and the rest of the world as a consequence of that.
This could start small, but soon she could find herself in a group of friends, relatives, and perhaps strangers, searching to understand what is happening or had happened for all of them to feel and appear this new way.