Small pimple on groin for 4 days...grew and became painful and very red...on bactrim (sulfamethoxazole and trimethoprim) now in case it's staph...is it rare to spread to blood.scared?

Yes: Spread to blood or septicemia is a very rare condition in healthy individuals. The Bactrim (sulfamethoxazole and trimethoprim) antibiotic treatment should also help to eradicate the staph infection

MRSA: It sounds as if you are being treated for furunculosis caused by a strain (USA 300) of Staphylococcus aureus that is resistant to methicillin (MRSA), and this is quite appropriate. It would be unusual to see this become a bloodstream infection, although nothing is impossible.