Confused! No period, negative tests

I've been seeing my boyfriend for about 6 months. Every time we have sex, he uses a condom. So, we use birth control and have protected sex. We are both stable adults, but not ready for kids yet.

My period always starts on a Friday afternoon and ends Tuesday morning. Very regular. 

It didn't come Friday, and by Saturday afternoon I was freaking out. I took a pregnacy test Saturday night and it came back negative. Didn't come Sunday or today (Monday). I took another test tonight. Again it was negative. I've read everything online about how stress, excessive exercise, weight loss/gain, etc. can cause one to not have a period. I wasn't going through any major stress prior to Friday. I was actually happy, with Thanksgivivg and spending time with family. Not having a period has caused me to stress. I haven't even been spotting...nothing. I've had cramps, and been really tired, like I usually am during my period. So I am having the symptoms, but no period. 

Anyone else have this happen to them? What happened? Should I be worried? Help!