5 days late and LH surge?

I am 5 days late for my period, but all last weekend I tested negative. I was supposed to start last Friday (09/16). I stopped testing cause I was sick of seeing the single line, and decided to wait till my period started. Still nothing. Two days ago I started testing my ovulation thinking maybe I didn’t actually ovulate and that’s why I’m late, but it’s the highest it’s ever been. I’ve taken the cheap Walmart tests and a few FRER, but nothing. I plan to take a digital this Friday cause it will be a full week since I’ve missed if my period doesn’t come. My question is, why is my LH suddenly starting to surge again?

I’m 18 DPO, btw!