I'm allergic to bactrim (sulfamethoxazole and trimethoprim) n just got prescribed sulfamethoxazole but i have severe shoulder pain and headache, it's an allergic reaction isn't it!

Likely: Bactrim (sulfamethoxazole and trimethoprim) allergy is usually a sulfa allergy so would think you would also react to sulfamethazole. Let your doctor know and be sure they are aware of the allergy!

UR allergic to sulfa: Bactrim (sulfamethoxazole and trimethoprim) contains sulfmethoxazole & trimethoprim. Most people allergic to Bactrim (sulfamethoxazole and trimethoprim) are allergic to the sulfa component. So your symptoms are probably related to sulfa. Your dr should prescribe another antibiotic, if you still need one. Suggest you make both your dr & your pharmacy aware of this.