4 Days Late

Samantha

So I am a little worried, I am 4 days late for my period. The first two days I had pretty bad diarrhea and back pain. I have taken 2 pregnancy tests and both have been negative. All I can think about is how my boyfriend and I had marathon sex with no condom about 14/15 days ago. No other symptoms other than being tired all of the time. Am I just freaking myself out or could this be real? I have never been late before.