I get constant headaches that don't seem to go away. I stopped taking my Paxil (paroxetine) prescription about 3 weeks ago could this be the cause of my headaches?

Serotonin DC. Abrupt discontinuation of some medications that work on serotonin can cause considerable problems in some people - not all. More typical symptoms include: malaise (washed out), disconnected feeling, nausea, irritability, insomnia, headaches, brain zaps, dizziness, and even confusion. They can begin immediately or if longer half-life, weeks later. Not life-threatening or cause of psychosis. Taper.
Rapid withdrawl paxi. Yes, that is a possibility. Stopping Paxil (paroxetine) and other anti-depressants suddenly can cause a great many symptoms, and headache is one of them. You should call your doctor who prescribed this medication and discuss your current problem.