Try coming at the number limit from the other side.
Rather than trying to think of a story you can condense to 100 words, which can feel as though you are having to cut out and sacrifice too much, try starting from a prompt such as a common phrase and play with it to see what you can grow to 100 words.
Someone has already pointed out that your question is getting on for 100 words, which should already you a feel for how much you can get across within that limit.
I thought that before I submitted this as an answer I should try it out for myself...
So, most stories are about relationships in some way, which might be why the phrase ‘It isn’t you’ popped into my head, then I thought about what if it was literal, what if somebody wasn’t themself? What if they’d been transformed somehow, what if they’d somehow become a monster and didn’t know… How could they not know, how could someone else know before they did? So I played with that a little to come up with something.
It's far from a perfect short story, but demonstrates how, in 100 words, you can start from a small notion and build up to something which hints at what went before, what happened 'off camera' and what might happen next.I went for cheesy word play, pulp horror and bad writing tropes, but you could go classier. Here's the short tale of when Chuck got handsy.
He stood, two arms full of logs, ‘‘Leaving’? You weren’t ‘leaving’ down by the fire ten minutes since when my hand…’ he gestured crudely. ‘What changed?’
Hannah backed towards the door, bag and coat spilling, all trembling fingers and thumbs, shaking her head desperately, ‘it isn’t you, …’
Gripping the firewood tightly, Chuck ran a hand through his hair as he reached out in supplication and confusion. ‘Babe, what? You meet someone while I was gettin’ logs?’ Menace shaded his voice as the hand at his back reached for the poker.
‘No, it isn’t you… Chuck only has two hands.