Almost positive OPK yesterday, line much fainter this morning?

My line was almost as dark as the control line yesterday afternoon. I took another test first thing this morning and the test line was quite a bit lighter than yesterday. Could that imply my ovulation window was actually sometime in between the two tests? Feel like I heard the window is only 8 hours, so I thought that could be plausible? However I also feel like I heard you’re not supposed to test with first morning urine, so maybe that’s why it was more negative?