Hi Samantha

This is a good question.  There can be many factors which can cause a missed period...such as pregnancy...an ovarian cyst....stress...illness...among others.  But as far as withdrawal symptoms...I have learned that with Effexor it seems anything goes.  This is a very difficult medication to wean from.  Your best bet is to ask your doctor if there could possibly be any link. 

In the meantime I found this forum where some patients who have taken Effexor and have weaned from it have experienced what you are experiencing with a missed period.

Here is information too from the NAMI website on side effects of Effexor:

Rare: Other side effects which may occur infrequently with venlafaxine include increased heart rate, low blood pressure, increased salivation, irregular menstrual cycle, increased frequency of urination, changes in taste, increased liver enzymes, increased bleeding (e.g. gums may bleed more easily), low sodium, and teeth grinding.

So it may be possible that the Effexor is causing this but to be on the safe side...you should probably contact your gynecologist to make sure it is not a gynecological problem.

I wish you the best of luck.  Write back to let us know how you are doing.