I missed a dose of effexor (venlafaxine) and did not end up taking it again for another 36 hours. very nauseas, and diarrhea. withdrawal symptoms?

Possibly: Serotonin withdrawal syndrome related to abrupt cessation of an ssri like medication could certainly cause these symptoms. Also, check a pregnancy test!

Yes!: Effexor (venlafaxine) has a short half-life and really needs to be taken every 24 hours (the xr form); or twice a day (the immediate release form). When you don't, you may have serotonin discontinuation symptoms: nausea, abd. Cramping, diarrhea, headache, balance or gait problems, sore joints or muscles, irritability. These symptoms go away within a couple of hours of taking the next dose.