OPK questions!

S

Here are 3 different test. First one was yesterday 5/15, second one is today 5/16 6:00 am (FMU), and the last one is this evening 5/16 around 6:00 pm. This is my first time using them and I’m wondering how long the positive opk should stay positive as the third one from this evening already seems a bit lighter than this mornings test. Also I am on CD 12.

If I BD’d last night and tonight am I still covered? Thank you! Any input and suggestions help!