I coach teens and weird things like this happen all the time. It can be any number of things- growth, hormones going haywire, stress, fatigue, starting to second guess themselves, or just being plain exhausted from everything going on in their lives as well as just in their own bodies.
I have one kid in particular who is going through something like this, a combination of the above as well as additional stress and pressure about competing for the first time with a lot of expectations (mostly ones she put on herself). Gymnastics has also come easily to her up to this point and she is just now running up against skills that are presenting a significant challenge, which I think could also be part of the frustrating fueled roadblock.
Just try to be patient and encouraging, she is probably getting frustrated with herself and if asked could probably not even begin to answer what's going on. It's very possible that she just "feels off," which is a completely legitimate reason in teens.