What will happen next?

OK so this girl was jealous of me because she liked my best friend, but now suddenly she’s becoming nice to me. Just after I’d resolved to try be nicer to her. Well, she wasn’t really NICE nice, but like she didn’t give me dirty looks like before. I’m still feeling unsettled though. How long will it last? Will I do something wrong again? I feel like she is bearing a grudge against me because of something I did a few months ago.  She promised she would never do it again, but she’s betrayed me twice before and I don’t always believe her promises. Now that that new girl is suddenly being nice, it makes me feel scared rather than relaxed. What will happen next? Why is she being nice all of a sudden? But most importantly, how long will it last?