Tramadol does not contain serotonin per se.

But it does act on serotonin receptors. This may be a function of the action it has on mu receptors (pain area) and other receptors like the kappa one.

Some opiates also do this. Oxycodone has quite a serotonin connection in some people.

Some neurotransmitter actions are cascades...initiated by one system and then a second one becomes active. Histamine in the brain is like this, initiating other systems.

The bottom line is that many drugs overlap onto other systems and have actions that complicate things for people.
Some of these are discovered during trials, but many take years and years to come forward, during post marketing.

Thank you for this thorough explanation. Sadly, one of my doctors Rx'd me flexeril while on the extended release formula of tramadol and within a day things were getting weird for me. After a couple of days it dawned on me to check the contents of it and thats when I realized what was happening. Even more sad was the fact that I brought it to the doctors attention. Now I just tell all of them that I won't touch anything with serotonin in it. That was a very scarry situation for me.

What is so bad about serotonin?

Too much serotonin is the issue. Where there is too much action on those receptors serious things can go wrong.

Normally we all have serotonin working for us. There are many types of receptors too, located all over the body. But when you add drugs into the picture, this sensitive system can get overloaded.
And it varies from person to person too. Some people just normally live with high serotonin levels, and those types would be more prone to serotonin syndrome. It is thought for example, psychologically that timid or careful people are high serotonin expressors. Contrasted with high dopamine people who are drawn to excitement and risk taking activities. You won't find many high serotonin types sky diving, for example!
Those with high serotonin systems, often get the worst side effects from SSRI antidepressants.

http://en.wikipedia.org/wiki/Serotonin_syndrome

90% or so of your serotonin is in the peripheral areas of the body. GI tract and platelets mostly.