34dd bra to small what is next size?

Try going with a 34E. Though, if you haven't been to a professional fitter, you might want to try wearing a 32F or 30G bra if you added inches to your under bust measurement if you measured yourself. Do not go up a band size! Most of the support comes from the band. A 30G cup is no different than a 34E cup, the band size is the only difference between the two. If you want to be sure of your size, just go to a professional fitter at a specialty Lingerie shop. The Bras there might be more expensive, but that doesn't mean you have to buy from there, unless you find out that your size is a size hard to find like a 28E (yes, it is possible, it is still small compared to 34D though).