My guess is that if the pharmacist thought there was any danger to your health because of the reaction, he would have urged you to call the doc.
If you do end up thinking you should call, don't worry about the substitute doc... you're not going to get a diagnosis from him, just tell him "hey, my regular doc started me on Prosac but I am having a bad reaction, should I stop taking it or just keep going until he gets back?" of course tell him a bit more then that... but you don't have to spill your guts to him or anything, just tell him about the reaction. Any other info he should get from your chart that your doc left behind for him. Hopefully it won't be a long conversation.
If you don't think there's anything dangerous about the reaction you are having, and you can deal with whatever discomfort it is causing until Monday, then it should be OK to wait. By now it's only another day anyway.

. Don't hesitate if it is causing you concern though.
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-- The world is what we make of it --
-- Dave
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