Is it normal to get mild petechia (a few small spots) after vigorous scratching? when i scratch chest or legs hard, they sometimes show up. 331 plts

Yes,this is: possible and your platelet count was normal..

Scratching causes it: Petechiae result from a minor bleed from a broken capillary blood vessel. The most common cause for petechiae limited to a specific area is local trauma. Scratching is a common cause. If your petechiae appeared in certain areas only after you started scratching that would be a likely cause. If that is not the case or if your problem persist or worsen see your physician for a specific diagnosis.