I got a message from the covid alert app that i was near someone who tested positive on sat 5 dec. i did grocery shopping then, kept away from others and wore a mask all the time. does this count as potential close contact requiring me to isolate?

Yes possible: It depends on the range, and if it is less than 6 feet it is considered a contact. The longer the duration of the contact, the more likely the risk. Watch for symptoms, rest and avoid contact with others while notifying your physician. Check local policies online.

Contact: Unless you were closer than 6 feet for 15 minutes, I don't consider that close enough contact to cause concern. You could get tested around Dec. 10th if you still have concerns.