What Causes Delayed Periods With Negative Result In HPT?

I do have a history of irregular menstrual cycles ever since I first got them. That changed after I had a miscarriage of my first child, due to a motorcycle accident back in August of 2016. My cycles were exact and normal. However, October 30th was the last day of my last cycle and on November 13th, 14th and lastly 29th were the days I had unprotected intercourse. I haven t had my cycle and with the calculations from an app call Ovia Pregnancy, right now (if I am) I d be 6 weeks pregnant but on November 21st, 27th and December 11th, I tested negative. Now based on my counting, from November 13th, tomorrow marks the 4th week of pregnancy so I was actually going to test again next week. My question is, am I pregnant? I don t know what to do. Many people had very obvious signs that signified they were pregnant but even in my last pregnancy, all I had was extreme fatigue and constant urination. I don t know what to do.