Implantation bleeding or nah...

I've been off the pill since the end of April, am supposed to have my period starting tomorrow. However, on Friday I started what I thought was my period but it was weird: one tampon would be soaked, the next would be completely clean. Sort of a darker red, brown color, stopped completely on Saturday around noon. Not typical for my period at all, although my body could still be adjusting from being off of the pill. Also, my husband and I had sex in the middle of my fertile window, so I'm wondering if I could be pregnant. I've taken 2 pregnancy tests and both are negative.