Positive OPK then negative?

Hey, I took an OPK and I got a good positive at around 2.30 today. We've been out all day and I've came back just now and tested again, and this time I'm only getting a faint line! I've attached photos but the second one is blurry! Should I be worried and am I still likely to get pregnant? This is our second month trying and I'm so so hoping everything's ok.

First one