I don't know what a "pre workout pill" is, but I rally doubt that it does anyone any good. What is usually healthy is exercise, not a bunch of pills, lotions and drinks to consume with it. It sounds as though there may have been some sort of ineraction between the Serdep and whatever is in the extra pill you took.
What is unfortunate, is that after doing well on Serdep for some time, you seem to have stopped it suddenly and abruptly : which is really a bad idea. With all such drugs, they should be stopped only gradually, with the advice of your doctor. Otherwise it can cause unpleasant symptoms of various sorts. These do reduce and go away, but of course it is best to avoid them.
So your unpleasant experience is indeed likely to be caused by the sudden stopping of the drug, and they should go away as you return to your usual blood levels of the drug.
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