My Dad Takes Plavix & Bumped His Forearm Last Week.

My dad takes Plavix & bumped his forearm last week. Had a bruise. Early this week we noticed a raised whitish looking bump in area of bruise that looks like it is filled with pus and it is hard as a rock. Also looks like there are other ones ready to pop out any time around it on same arm. Cause for alarm? His father died as a result of Melanoma. Just worried.