OPK Confusion

This is my first month using opk. I have been testing every day since the last day of my period, twice a day, because I had no idea of my FW dates. I’ve been testing once first thing in the morning and once when I get home from work. I drink water alllll day long so I’m assuming the evening tests aren’t as reliable as the morning tests.

Today is CD 19 for me and I FINALLY got a positive O this morning (7am). I was starting to worry something was wrong. The night before my positive o, it was close to positive. Before that, barely there lines twice a day. Today when I tested again after work (7pm) today, I got absolutely nothing.

Being my first month I just want to make sure this is normal and I’m not doing or reading something incorrectly. I BD last night with my almost positive and will keep at it!