2 wisdom teeth(left) removed 3 weeks ago.when brushing teeth,hand slipped,hit the back of cheek,started feeling some numbness in lower jaw.worrisome?

Wisdom teeth: After extraction 3 weeks ago, numbness is highly unusual. See the doctor who removed the teeth. Otherwise, a visit to an oral surgeon for evaluation is in order

Numbness: You may have inadvertently bumped the nerve. It's not worrisome unless the numbness does not resolve itself. If the numbness remains longer,than 7-10 days, have it examined by your dentist or an oral surgeon.

Probably Nothing: See how it feels in a week. If you are still concerned, see your dentist.