I am also very very regular exactly 28 day cycle. When I was trying to concieve my daughter and would be a week late I would take a test and the line would be very very faint you'd really have to look hard which would mean it picked up a pregnancy hormone and I was pregnant. But... I would miscarry a few days later and have horrible cramps and all that..
Then finally after that happend about 5x on me I took a pregnancy test the line was faint, and would take another test a few days later and it would be a little darker and so on and finally I had me a baby.
There's a million reasons why that could be happening to you. I'm not saying your miscarrying but it is a possibility. If I wouldn't have taken so many pregnancy tests I honestly may not have known I was miscarrying ang just assumed I was a week or two late with alot of cramping. If I wasn't trying to get pregnant in the first place and paying particular attention to everything I would have thought nothing of it.
You can take apart a pregnancy test and just take the piece of strip that has the lines if your pregnant or not and hold it up to a light. If there are two lines you are pregnant. Sometimes you have to really look hard for a faint line.