Bra gurus...

[LittleMachine]

My weight has been up and down, so I measured myself (came up with 30D/DD) and went to get fitted. Went to Debenhams. Our local one has always been pretty good. They measured me as a 30D.

However, she sat the blackstrap really high on my back. Does that sound right? I don't remember that ever happening before.
The bras I bought felt comfortable and looked good when I tried them on. The one I have on today felt great until a few hours into the day, when it started rubbing where my armpit meets my boob - where the strap meets the cup basically.
What has gone wrong?!

The backstrap should be horizontal. If you're measuring at a 30 it's possible she's overtightened the straps which has pulled the back up or it could be that you need a 28 in that style

Rubbing under the armpits may be a style issue as some bras have taller or wider cups than others

I think you're right. Dammit. It's a plunge type and I'm better with balconettes. I really like it as well.
I tried a few 28s but they were all too small. Thanks SC.