Oh, for me it's the subtle little things that get me every time. People forget that little things together amount to big issues and the trans one specifically is one that most people have been dealing with for a while and usually has a lot of strong emotion attached to it. Sometimes you're in the zone, content, or at least oblivious to trans things and one thing can trigger dysphoria and topple the house of cards.
It's also kind-of easier when the words are direct--they have the chance to hurt more--but they can be dealt with in time. Little comments fester into wonderings, pick insecurities, and leaves your mind free to assume any number of things about the incident.
Ergo:
>"Why do you want a haircut? So you can look like a man?"
Can spawn into
>Why would she think I wanted that?
>Does she remember about the trans issue?
>How did she take it?
Which could have branches like
>Clearly not very well if she can't even handle a haircut.
>She won't understand
Etc. etc. etc. But I know the feeling. It sucks that little things can be triggers but if you're stressed it happens. Your grandmother probably wasn't trying to be hurtful--a lot of people are uninformed about trans issues so she probably wouldn't know any better. That's all that matters. c: If she was, it's still probably an issue of not understanding. I'm sure you can get through it.

Feel better though, yeah?