Can anyone help me with bra sizing?

[Raveon2000]

Obviously I should go and get a proper fitting, but just curious as to why my back measurement is 30 inch but a 32 bra feels like I can hardly breath?
Also a 34DD feels comfy but it's not flat to my chest in the middle between breasts, so I know it's not fitting properly.
Just tried bras in 32E but they are far too tight!

So are you 32F?

28FF or 30F, depending on the brand.

Update:
My bras came from Bravissimo and the 32F was a nice fit. I did the online live measuring and she had suggested 32F 30FF.
I've since been measured in M&S and she put the tape measure round my band and said 'you are measuring a 34' (that was 30 inch, 29 inch pulled tight) but then I tried on their 32E and she said that was a good fit for me.
So the moral of the story is every shop has a different measuring system!
Any way got 4 decent supportive bras now and a good sports bra