What Causes A Penis Shaft Spot?

I played 4 games of Hockey yesterday. I wore a jock strap that was fairly restricting. When I got home I noticed a dime sized Purplish red spot about half way up the shaft on the top of my penis. Do you think this was caused by the restriction, or sweat or should I get tested to see if My girlfriend is cheating and gave me an STD? Thank you in advance!