Seretonin syndrome is a state in which there is excess seretonin in the body. This is often a result of taking a combination of drugs which can increase seretonin levels, including some pain medication (e.g. your tramadol), antidepressants, some illicit drugs and even supplements. Most cases are self-limiting and can stop after stopping the drugs(s) that are affect seretonin levels, but others can be life threatening and require hospitalisation and administration of drugs that block the action of seretonin.
Symptoms of seretonin syndrome include confusion, shivering, diarrhoea, heavy sweating, headache, vision changes, hyperactivity, flushing, increases in heart rate and fever. Note: you don't have to have all of these symptoms to be suffering from seretonin syndrome.
Reference: Seretonin Syndrome, Australian Prescriber, June 2001