While not having early cycle temps isn't ideal, I think it's still pretty obvious that you most likely Oed on CD18. Many women experience what is called a "fallback rise" where 2DPO their temp dips back down below the coverline. It's hard to know for sure if that is what is going on because you don't have 6 temps prior to O in order to draw a coverline. The next couple of days/temps will tell, though! Good luck!