implantation bleeding?

Ch
Me and my husband are not TTC yet, we are waiting until May. I went off the pill in January and we have been using the withdrawal method. I was using cheap OPKs from eBay for this cycle to track and nothing, not even a faint line. On CD16 I bought cleablue digital ones, was going to use them next month but decided to give one a go. It said low fertility. So, I figured I'd missed ovulation and I would use them next month. On Friday night (CD18) we DTD and I figured he didn't really have to pull out, the chances were so small. Today (CD20) I have spotting, which never happens to me. I have a 29 day cycle. Could it be implantation bleeding?