Changes to period

My periods have been fairly regular since I stopped taking the pill over a year ago, my cycle length is 29.4 days on average. The past two periods though have been between 3 days and a week early, and we're ttc so this has put our ovulation dates totally off :( Also, I've been experiencing a totally new period symptom, heartburn, on both my previous periods, before and during. I've never had heartburn before thus but now it seems like it's becoming a regular thing, and I don't think it's linked to anything I've eaten, since it coincides with my period. Any ideas why this might be happening?