(New to OPKs) Why does OPKs fluctuate??

I'm currently new to using OPKs. I just want to know when I would be ovulating and I want to understand why my opk result differs.

I'm currently on my CD7, and my cycle usually lasts for 30+ days. Before, period usually starts on the 1st week of the month, but back in June I had to take Plan B which eventually messed up my cycle. That's why in August, my period started on the 2nd week of the month and same goes for this month of September.

I first used opk on the 5th day of my cycle (Sept. 13,) and the result showed negative however it was a bit dark.

The next day (Sept. 14), I took opk twice during the day which showed both negative however I think it was a bit dark considering that it was only the 6th day of my cycle. This got me confused because I just finished my period and I've always had a long cycle.

I took an opk today (Sept. 15) and the results were so negative. There's hardly a second line and I was wondering why the result changed so quickly.

Sorry for the long post, but can someone educate me on this? I just wanna know why. Thank you and I appreciate the response.