I woke up with an itchy rash all over my torso. Yesterday just finished a 10 day round of bactrim (sulfamethoxazole and trimethoprim). Could this be allergic reaction so late?

Yes. Drug rash can happen any time from within a few minutes to 10 days after a dose.
Sulfa dermatitis. It could be. Sulfa reactions especially those involving the skin are often delayed. There is no fda approved skin or blood test available for confirmation. Have it checked and documented. If no other obvious diagnosis would avoid sulfa antibiotics.