Have you been using them all month long? Was yesterday the darkest line you had? What brand are you using? If you had your period on the 10th, and you have 31-34 day cycles, it's still early for ovulation. So, getting a brighter line one day and a weaker line the next isn't unusual, since we all have LH throughout our cycles, and it waxes and wanes. You can even have multiple surges, but this is sometimes a sign of PCOS. Are you temping? Checking CM and CP? How many months have you been TTC, and is this your first month of OPKs? If possible, post pics of the OPKs... they can be tricky to read and lots of ladies here have lots of experience with them! GL and FXed for you!