normally pretty regular but now..

My husband and I are trying to conceive. We charted my ovualation and had sex on the days I was fertile. On day 21 of my cycle, I started having brown spotting and thought maybe it was implantation bleeding. That night though it was heavier and I had to use a tampon. I thought it was my period because I used a tampon all day Monday and by Tuesday afternoon it had stopped. It started again Wednesday afternoon. What is going on? I am normally pretty regular (27-29 days) and my period normally lasts approx 5 days and is very heavy in beginning. It was not very heavy on Monday but did soak a tampon.