Cup size confusion

Tyffany • Mom, wife & nurse...pregnant with baby #2!!
I was a 38B before pregnancy.   I was curious so I looked up how to figure out my cup size.  It is telling me I'm a 40D!! Now, they are fuller, but it does not seem like I'm two cups sizes bigger already, I'm only 18 weeks.  Is going to the store and trying them the only way to know for sure?  Has anyone gone two cups this quick?