Trying to determine LMP & how far along I am?

Hi ladies! So I got my BFP on March 17, but I tested 2 days earlier and it was negative. I’m just trying to get an idea of how far along I am before I go to my doctor’s. My cycles were really irregular before, and I have an ovarian cyst that wasn’t helping things. One cycle started on Jan 31, and the other began on Feb 24. Any ideas? My first appt is March 29 with a nurse, so I’m guessing the first appt with a doctor won’t be until the first 2 weeks of April. Any insight would be greatly appreciated! ❤️