Funny you mention aspirin. Aspirin probably is the biggest killer of all medications, causing about 40-50 thousand deaths a year once it became popular through advertising. The FDA was helpless, however, because when the Food and Drug Act was passed, aspirin was grandfathered in as GRAS -- generally recognized as safe. Of course, this isn't relevant to the original poster, just a warning that few of us know much about what we're taking.
My understanding of serotonin syndrome, which is a overdose of serotonin in the body, is that you'd know if if you had it -- you'd probably be in the ER. As Il says, it's very rare.
Its an extremely rare adverse side effect (ones that are statistically rare) from anti-depressents. The entry in Wikipedia appears to be accurate and has accurate links. If you are not experiencing anything like that best not to overly frighten yourself about a statistically rare side effect. If you are having any unusual side effects of any kind best to describe what they are as it would unlikely be that but if you ever experience any side effects that are affecting your health report them to your doctor right away. Every medication, including asprin has adverse side effects so if everything is going well I wouldn't get overly concerned about having that one happen.