Sure, it's possible to ovulate on CD17 and have an 18-day LP. It's also possible to ovulate earlier than your norm and have a shorter than usual cycle. It's also very possible to have fertile CM and cramps and not be ovulating right now.
Unless you and your DH have the stamina to have sex every other day throughout your cycle, I'd suggest charting your BBT or at least using OPKs to help you figure out when you're fertile.