cant wait to find out my due date!

not sure if I got pregnant in October or November since I missed my period at the end of October which has never happened! I didnt get a positive pregnancy test until November 16th though so I'm thinking I may have just ovulated late. (got positive opk from nov 3rd to 5th) So I'm thinking my due date could either be beginning of July or end of July. Has anyone ever experienced this? Are they just going to go by my last period for my due date at my first appt until I have an ultra sound?