If you mean you peed on a stick let's say 5 hours ago and the lines were dark and now they are light then my advice is read the instructions because pretty much every pregnancy test instuction says don't read results once it's passed like 10 minutes or something because they might not be accurate....
The higher the hcg level the lighter the test becomes over time. If you pee in a cup add a bit of water to it and re dip the test it will brighten up. Happened to me and i did this and it was brighter. No worries
Are you using the same brand of tests each time?
No. Tests aren't that sensitive to hcg levels. Its either the test or maybe the time of day you took each test