Wow... Some of the answers on here.
Firstly, I get migraines, they're not always catastrophically bad, but they're diagnosed by a GP as migraines and respond to triptans like migraines do. Some people get migraines that don't even hurt! Just vision symptoms.
BUT! Some people (like my son), get pain that is catastrophically awful. I have occasionally had those myself - once in hospital - and all I did was lie completely rigid, cry, and keep my eyes shut the whole time - even when a doctor was speaking to me.
If I'd been at home alone and the pain was that kind of severe I might have tried to call for help and been incoherent too.
If I received a text like that I'd take the person at their word and respond. Even if it was just to call or text back and say "I can't come right now but I will be with you asap".
And why can't OP have recovered a little since the initial pain was that bad? Maybe she panicked at first and now she's a bit less in pain and calmer and wondering if she just got blanked by her sister?
To be honest though OP, it could just as easily be that she didn't get the message.
I wouldn't worry about it too much - or all this criticism either. Anyone would think you killed your sister's cat by some of these reactions. 
There is some good advice though - keep meds close by at all times.
Hope you feel better soon.