Serotonin syndrome or something else?

I have been taking zoloft 100mg for 5 years, about 3 months ago i quit cold turkey for about a week and due to horrible withdraw symptoms i got back on it at 50mg. a month later i noticed it wasnt working well so i bumped it up to 100mg. its been about 3 weeks since i bumped up the miligrams and i am getting dizzy, and headaces and my heart is racing after taking my zoloft. is it possible that i am experiencing mild serotonin syndrome or is it something else?