Ovulation immediately after period

I started my period 9/6. I've had bleeding until 9/15 then spotting until 9/16 around midnight.

I got a positive OPK today (9/17) My husband and I had sex with Preseed (not rough; ds) and the light pink to red spotting came back. Not enough for a pad or tampon and only when I wipe.

Can anyone help? Could ovulation happen immediately after period?

I don't have regular periods. I have one every two weeks.