How Should A Tricep Spot With Hardness And Blisters Be Treated?

I was stung by a eastern yellow jacket 48 hours ago. Immediate reaction was minimal. 24 hours ago arm felt like I had a flu shot. Today a hard spot under tricep about the size of a half dollar with an area the size of a quarter developing small blisters. Does this require any medical attention? I was stung a month ago by a yellow jacket in the shin. It became very itchy and later developed scabs due to my constant scratching....but no other after affects.