Spotting After Ovulation

I am 37 with a 7 year old. I became pregnant the first month I tried and had a missed miscarriage in november that resulted in a d&c. I have been trying to get pregnant since january. This month I began charting my temp. It appears that I ovulated on the 21st. I had the ovulation pains and it was the right time -- with a drop in temperature followed the next day by a sharp rise. However, two days after ovulation, I began spotting. Not enough to be noticeable except while wiping. It has lasted three days. I'm really concerned because it is obviously another month of not being pregnant. Has anyone experienced this and what is causing the spotting?