10DPO FMU Possible Positive Pregnancy Test

I have been testing with a dollar store test every day twice a day for the past 2 days. Yesterday morning I had a very visible line, but in the afternoon it wasn't very noticeable but there. This morning I tested again with FMU and I got another faint squinter line, however, it doesn't seem to be as dark as yesterday mornings test. It takes 48-72 hours for HCG levels to double which could explain why my tests aren't getting that darker. I have also read that some tests have more dye than other tests so sometimes the lines can get lighter. I also tested this afternoon around 4pm with a cheap test and there was barely a line again. I could hardly tell if it was an evap or not. Now it does say on the instructions of this test that first morning urine is recommended which would explain why I get positives in the morning and possible negatives at night.