I'm guessing that she's not taking ages to text back, you're actually texting her expecting yourself to be a priority, when really she's ignoring them for a while so that her boyfriend doesn't get suspicious and dump her.
If she was interested in you as a relationship, she'd have done the right thing (well, next best right thing I guess) and dumped him when she got home. Nope, she's going to act like it never happened, and you're going to suffer because she's just not that into you.
What was it my wife said long ago to dismiss me when she slept w/ me while she was dating someone else... "It has been remarked to me by more than one woman that once you boff a boy it's difficult to get him to stop trying to re-cross that line...and for him to continue to attempt to do so would be unwise." Long story short years later we started dating again, but at the time it was a very frank, honest, and cruel dismissal. Suck it up, you're not her boyfriend, and very likely you never will be. Either you can maintain your own moral high ground and not sleep with her again because she is in a relationship, or you can do whatever you want, but if that's the case why are you here asking us questions?
"Well, then," the Cat went on, "you see a dog growls when it's angry, and wags its tail when it's pleased. Now I growl when I'm pleased, and wag my tail when I'm angry. Therefore I'm mad."