What Causes Small Red Spots On The Upper Body?

Ive been taking bactrim for an abscess i got. Im almost through the prescription of 40 pills twice a day. yesterday i was sitting in the sun for about half hour and got burnt on my shoulders. then today at work it was very itchy and i scratched a few times. i then noticed little red spots spread down my arm, up my neck, and a little bit on my upper chest. can you help me out to what this rash is?