Ummm...serotonin syndrome?

I'm currently taking Ultram (2-50mg pills every six hours for pain) and Celexa (40mg). I'm also taking Propranolol for migraines (20mg twice a day). My doctor prescribed both Ultram and Propranolol knowing I was taking Celexa, but the pharmacist mentioned seratonin syndrome when I picked up my prescriptions...what is that?