How come my heart beat fast - I took vicodin - 3 hours later Paxil (paroxetine) and heart started racing?

No idea. There are "idiosyncratic" reactions to drugs, which mean that they are not common and only happen to certain susceptible individuals, usually for reasons that cannot be discovered. Ssri drugs like Paxil (paroxetine) can occasionally produce heart arrhythmias, and you should talk to your doctor. The vicodin is unlikely to have been responsible, although if the vicodin wasn't legally prescribed, anxiety might.