How to work out my bra size?

Last time I got measured in a shop I was half way between a 32 and a 34 back size and was a D cup size. I've been wearing 32d bras with a back extender. Since then I've put weight on my hips and boobs (which I don't mind lol) and I've worked out by guidelines on the internet that my backs a 34 now, but I can't work out my cup size. According to the internet I should be a 34A, but my boobs pour out of 32D bras, so I don't think thats right. What am I doing wrong?