Skipped period-BFN test...what's going on?

I've been TTC for 3 months now. I was supposed to start my period last Monday but didn't. I definitely had all the symptoms of a normal period of mine and I'm usually right on the money with my cycle occurring every month. Today, I took 2 pregnancy tests and it said negative on both. Should I call my doctor or is this normal? My girlfriend said it could be because I haven't gone to the gym at all this week like I normally do. Advice? Help? Guidance? Thanks!