BBT? I just don’t get it..

I know charting every day ensures more accuracy, but I lost my thermometer more than once and kind of screwed myself a bit. But if it remains high after ovulation does that mean pregnancy? I’ve 4 days till AF shows up. Tested 2 days ago with a BFN, but between yesterday and today, major diarrhea, mood swings, fatigue, back aches, bloating, mild one sided cramp, and boobs heavy and sore. I’m about to do another test tomorrow. Just curious what y’all think. Pms and pregnancy symptoms should NOT be so identical when TTC 😡