I was just talking about this with someone the other day. If I can't know how to take someone's words at face value I can get really tense about being around that person. If I'm being teased I have to know I am, rather than feeling set up to be laughed at if I didn't realise the other person was messing with me. If you used to be really jokey and playful and are now acting differently I can see how that could start this sort of tension -- if it were me I'd be thinking you were playing some sort of game with me and I would be suspicious, especially since pranks used to be a specialty of yours.
I don't think you should try to be anyone you're not, and while she's with this other person just consistently be her sincere friend. While she's with someone else you have a bit of time to try and regain the friendship.