There could be any number of reasons, but knowing neither you nor your girlfriend nor your exes, there's little information to go on. How often does this happen? How do you respond? Is there ever a 'horny' text, then sex later? Does sex only ever happen without a 'horny' text? Do you feel a problem with your sex life in general - it seems so.
What does your girlfriend do during the day? Is she on medication? Not forgetting, feeling horny but not being able to sate it is frustrating, and such frustration itself is an aphrodisiac. When you're together, sex is possible, so there's no frustration.
So yeah, talk to her about it. It's obviously bothering you. It seems you see a 'horny' text as something like a mix of foreplay and a promise of sex - ask her if that's how she intends it at the time. It also seems there is a wider concern than the text matter.
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Of course, it's probably quite a bit more complicated than that.
You know sometimes, between the dames and the horses, I don't even know why I put my hat on.