Ovulation strip questions

Hi guys this is my first month using ovulation tests and I am so confused by them! Is there a pattern they follow? I assumed it would gradually get darker until it was positive but mine have been kind of back and fourth in darkness to lightness all month. This is the darkest it has gotten but it has been this way for the last 3 days. It was this dark once on the 19th so i thought that might have been the positive when it got lighter after that but now it went back to this when i should be having my period IF the 19th was the positive. Just looking for someone with more experience to tell me what I should be expecting bc I am starting to drive myself insane. Thanks in advance ladies!