Why do underwires ALWAYS cut into me?
It is because it is the wrong shape. You can have a bra that fits however if the shape is wrong then the bra just wont work, it wont support you and it will be very uncomfortable!
This happens alot in smaller cups as all commercial lingerie companies have the same shape, their measurements are incorrect and they are not tailoring them to a woman's body. They need to move the cups a little further apart and that will stop them digging into you. If you try a French, Italian or even UK design you can see and feel the difference.
If you read my post on It's All About The Shape it will help you.
I am having problems with rashes under my breasts from heat/sweat and was wondering if you could recommend a bra or why this is happening?
From my experience I do see alot of women who get rashes under their breasts and the majority of the time it is due to an incorrect fit. If you are not getting lift from your bra then it will rub and as you mention above from heat and sweat it will cause a rash. It is because there is no air getting through.
What I would like you to do is; with your bra on look in the mirror side on, are your breasts still going South? If so then you are not getting the lift you need from your bra.
This is if the rash is directly under your breasts.
I would recommend you re measure yourself. You can read my How to Measure Your Bra Size and that will give you your bra size.
Meanwhile go to your chemist and get a cream, then you need to be fitted correctly.