Bras - another one here who has gone from a 32 to a 36-38 with the same cup size. I can't believe it and I have to admit I've always cried about bras too!
Weareboats I used to measure myself as i hated buying bras so much!
I think this is where the mothercare mistake came from - In the past (10 years ago) you used to measure as follows - add the 4 inches to your actual rib cage measurement if you measured even (eg you measured 36 so wore a 40 bra), or add 5 if you measured odd (eg you measured 35 so wore a 40 bra). Then you used to measure around the widest part of your boobs and if they were the same size as the added 4/5 then you needed an A, 1 inch larger, B and so on... I.e. 35 or 36 rib, with 41 boob was a 40B!
Now it has completely changed and that's why I went through another repeated phase of crying, as the bra size will match your actual measurement round your ribs... eg you measure 36 on the ribs, so wear a 36 bra. I think the cup is therefore based on the true difference, ie 1 inch wider on the boobs is an A, 2 inches a B etc etc. So in this case 36 on the ribs, 41 on the boobs would give you a 36DD - so it sounds like you picked the perfect bra, but she measured you the old fashioned way which is both confusing and upsetting - and I've def been there too!
The only thing I did learn at mothercare that they didn't think of in M&S is that when we're pregnant, we should buy a bra to fit on the tightest hooks, so we have room to grow as baby grows and our ribs get bigger. Afterwards, when breast feeding our ribs should go back down a bit (there is hope joskar), so buy a bra that fits on the looser setting so as you shrink & the bra stretches with age, it continues to fit well.
Hope that helps, completely understand how you are feeling, I have always cried when buying bras until I found bravissimo a few years ago, but since being pregnant I've had the same experience. 