Muscle failure doing pullovers

Hi. I'm a beginner doing the FB30 program. While doing pullovers lying on the back, I felt my right shoulder suddenly relax at the point of maximum extension and I dropped the dumbells behind my head. Since then I've had a pain in the right shoulder for 2 weeks that's gotten better but not gone away yet.

Is this normal? Can too much weight cause a muscle to suddenly relax completely?