How to determine bra size?

I had a bra sizing done and the manager/top bra sizer told me that she sized me at 34dd. Its mostly for around, as my breasts might only be 34d. A 34d is too small around and I can't even get it fastened on the last one! The 34dd fits perfectly around, I put it on the 2nd hook. But the cups pucker, just a tiny bit. I'm a very petite person. A 36 c is really tight around my back, but the cups fit ok (one pops out the top). The 34dd is the only one that covers the cups completely. What is wrong, and what size am I?