First things first, are you sure you are a 34dd? Have you measured in the tight underbust, then (with nothing on) bend over so your body is parallel to the floor and measure loosely round the largest part. Underbust is your band size, over bust difference in inches is the cup - Bra Guru.
I usually size up in the band for sports bras as they are so tight. My current favourite running bra is a M&S crop top style one (bugger to get on) and strangely a tescos bargain one
I have measured using the mn method and currently that's correct - it's going to change though! my most supportive one is the one I wore today and it is rather uncomfortable under bust so I know I need to try something better.
I have a newly opened sports direct near me, so thinking of something from there?
the shoes analogy is probably right - my band seems to have become tighter though - too much work on my lats maybe . I don't think I could even do my normal size band up on a shock absorber Run - I am usually 32DD, size 8.
Watch the adjustable hooks on the back of the shock absorber run bra- one has ahem 'come undone' twice on me and once on my friend. Mine was in the gym, leaving me looking very lopsided in the mirror, and hers in a race, leaving her crouching behind a hedge taking her top off I put a couple of stitches in all my bras to keep everything secure!!