Do you think it's possible she has your number blocked?

You need to move on. She's not the one for you. They're called smartphones for a reason -- they prevent you from wasting your time stupidly in doomed relationships. Trust the AI!

Unless you or she live in some fringe area where the reception is worse than spotty, phones just don't do this. They aren't smart enough (yet) to pick and choose, nor do they psychopathicly play with people by intermittently blocking only one specific user. I'm sorry to say, but as Dathbane suggested, the issue is at the other end.

I had a phone that did this same stuff! not sure if you've got iPhone or Android (I have Android) so anyways I had to go into settings and force stop my texting app, clear any cache data and that seemed to help with the issue.

despite what people think phones do do this. you're going to need to to clear any cashed data ,clear your cookies out, you may need to even go as far as to reset your phone to default settings before putting your preferences back. I used to have a phone that did that all the time it wouldn't accept texts or calls from people where I had a lot of saved text messages. so I had to remove the texting app redownload it and then everything was fine.

The most likely explanation is that girlfriend has blocked your number. She might or might not be have intended to do that or might have done it by accident. Get her to check your contact in her phone carefully to see if she accidentally blocked you.