Negative pregnancy tests.

I took a pregnancy test in the evening and it turned out to be negative I did fmu the next day . Last months period was July 4th. My period is now 6 days late, my boobs and nipples have been sore for about 2 weeks, I thought I was starting my period on expected period day but it was just cramps (no bleeding) , ive been getting lower back aches and cramps, and now I’m extremely tired. Not sure what’s going on I’ve been having pregnancy symptoms with negative tests! Anyone with a similar experience?