False positive?

Gi
Hello, I am currently 3 days late on my AF. I have taken about 5 pregnancy test since I was 10 dpo up until this morning and all have said negative. I've had mild cramping that I'd normally say were period cramps for about 5 days and have had mild heartburn at night and have been a little emotional (unusual for me regardless of period). 
So I was emptying out the trash and I was going to hide the negative tests from me MIL who is staying with us and then I noticed one of the tests appeared to have a light pinkish grey line on the positive area but it was from the next day (I took it last night before bed). Is this why they say not to look at the test after 10 minutes or could this mean that I could be pregnant? It was one of the Walmart 88 cent tests. I tried to take another but that one didn't even register a control line. Grrr.