Should I think I'm only feeling happy because I'm texting my ex again
Do you not expect to feel happy when texting your ex?
No, I don't expect that at all.
If you think you shouldn't be happy texting your ex then there is probably a reason. If things ended badly then maybe you should move on.
Nah I don't care. They are texting back.
If you're happy to be texting them, I don't see anything wrong with it. People do stay in communication with an ex from time to time. Sometimes they even try dating again.