My PCP Has Prescribed Tramadol For My Pain Because I

My PCP has prescribed Tramadol for my pain because I have had a gastric by-pass. I was told that taking aspirin was very bad for my stomach....Ok, I can understand. My sister-in-law had surgery and she gave me the remainder of her oxycodone. i only had a few, but in an emergency, they worked. I still have them but i am actually afraid to continue because of addiction...??? ANYWAY, the question is...what should I expect for side effects with Tramadol?