Bleeding during ovulation

I have been off depo since August and had a Mirena from September thru January. My husband and I have been having unprotected sex since. We aren't really trying, but definitely not using any method of protection and are not opposed to getting pregnant. I've been tracking my cycle for the last two months and my period finally seemed to be getting back on track this last cycle, until I started bleeding the first day I should have ovulated. It lasted for about 3.5 days. Does this mean my cycle is still out of whack from the hormones, or possibility of pregnancy due to implantation bleeding?