Been on Sertraline for about 12 years now...Could this be the cause of my chronic hives that I've been having since October? Ruled out foods and stuff.

Unlikely. While it is possible, it is unlikely after all this time. If your doctor cannot find the cause, ask him/her for a referral.
Medication effect. Side effects of Sertraline can be Urticaria (chronic hives), rash, increased sweating, and Acne. Avoid known triggers-stress, alcohol, pain relievers, heat, cold, exertion. See Dermatologist. See Psychiatrist for Diagnosis; discuss benefits and side effects of Sertraline, decreasing the dose; Therapy for stresses in your life, to develop coping skills, to resolve unpleasant feelings and conflicts.