needing help..

I was supposed to ovulate on March 12, but I wasn’t aware because I downloaded this app just recently. Well my boyfriend and i had sex on March 11, and then again a few days later. I was scheduled to get my period on March 26, but have not. I tested on March 26-negative. So i tested again on March 31-negative. I’ve been having sore breasts for the past few days, light cramping (on and off) and lower back pain (on and off). So i decided to test again-came back negative. Anyone else have this problem and still ended up pregnant? Any ideas on what I should do?