Help?

Okay ladies- didn’t feel like my OPKs were getting anywhere so after CD15 I felt defeated and quit. Terrible headache CD18 and decided to test- came back hours later to the bottom test being almost positive and took another which was completely negative. Is it possible that was the tail end of my surge from day before or I had a short surge that day? I know it’s not quite positive but that’s the darkest mine have ever been. Can’t temp because I work night shift and sleep schedule is erratic. So confused 😂