Can the OPKs be wrong? Please help

Here's the story...I'd really love some input! I had a 2nd trimester lost. I finally got a negative pregnancy test 6.5 weeks after my d&c. About 1.5 weeks after that I had a period (start 7/4). So I wait a couple of weeks and now I've been using OPKs (digital) for 13 days and never got a smiley. I look at the strips before I throw them away and there was a few days were the lines look like a match to me but like I said, no smiley. Now my test line is starting to fad. Do you think some digitals miss the surge for some women? Or what's going on?