Filling broke on upper tooth opp where lower filling repair done 3 weeks ago. Had gone back bc of sensitivity and unable to floss lower. Dds didn't c!?

See a dentis. Probably will need another filling on the top tooth that broke. See a dentist to look at your teeth.
Occlusion. Sounds like the occlusion meeting of the upper and lower teeth were not proper and caused the restoration to break. I'm not sure about the rest of your question.