Is she mad at me? What does this mean?

I recently got back in touch with one of my old friends (we grew apart). Well she keeps texting me every weekend and asking me if I want to FaceTime, which is fine but it is starting to become a bit irritating. I don't hat her or anything, but since we grew apart and go to different schools it's hard to talk about stuff and common interests because we are so different now. The last time we FaceTimed, we both didn't really know what to talk about and it was awkward. Anyways, I have these exams at school coming up next week and they are super important (they are each worth 50% of my overall grad in each subject) so there is a lot of pressure on me! I am planning on studying all weekend to prepare more for the exams. Well, today my friend texted me asking if I wanted to FaceTime, but I hold her I couldn't because I was studying. She replied back with "mhm...-_-". Does she not believe e or something? Is she mad at me? How should I respond?