Pristiq and period?

I've started taking pristiq (anti depressant) for a month now and my period is one week late. I've had unprotected sex and thought I was pregnant (I would be 5 weeks pregnant) since I was having all of the symptoms; backache, nausea etc and it came out negative! I'm begginning to wonder if my medication has stopped me from getting my period or if my medication is creating a false positive? Has anybody else had experience with pristiq? Any comments or advice are greatly appreciated xx