Another spotting question

Today is CD 28. I had some really light spotting on CD 16, thought maybe it was from O. Now I've had a little spotting again on CDs 25, 26 and 27. It's very light, basically one little spot of brown each day. I've also been getting BFNs so I'm expecting AF within the next few days. This is unusual for me. Now, this is my first full cycle since my d&c, maybe its related? However, I've had two early m/c this year, then with my third pregnancy all my levels were ok until I started spotting at 6 weeks, then we discovered my progesterone was dropping (ended up with d&c at 8w3d). Doc and I decided that since my levels were ok in the beginning last time I would only start supplements if my progesterone was low if I got a BFP, but with having the spotting this month now I'm nervous that its too low to start with. If it were you would you ask for a CD 21 progesterone check?? Sorry this got so long.