How rare is serotonin syndrome? What is the likleyhood of me getting it?

I am 15 and I take 50mg of Zoloft a day but it doesn't affect me like most people because I am unfortunately obese (trying to lose the weight though). I have been prescribed to take 50mg of Ultram as needed every 4-6 hours for a pain in my lower left side, and Lortab 5 when the pain is really bad. I've only taken 2 Ultrams today and I don't take Zoloft until the evening. I didn't know about the serotonin syndrome until after I took the second one. I'm really scared now. Am I okay?