Eek.... I'd hate to be in your boat... keep us updated. Her irrational behavior could be homesickness or it could be "junky-like behavior" (a phrase my friends and I use to describe behavior that is that of a junky)
But the red flags in this are saying things like " you and dad are making me want to do drugs."
Sounds like the manipulative talk of a junky. Also if something like that sends her into "wanting to do drugs" mode... Clearly she's not done with rehab..
Also, get into alanon or something similar, as the experiences of others would be great help.
Also, Also, you're her mother, don't let your child talk to you that way. I know you're afraid to push her away and not be perceived as supportive, but don't enable her behavior in any way. She's in rehab because she eff'd up and got hooked on drugs, she is in no position to be telling you what to do. If she was 16 would you let her talk to you that way? well she's only about 6 years older than that now. Dig deep down inside and find your mommy voice... That terrifying pitch mothers use to freeze children who are about to stick their hands in the toaster...