Itchy rash on and under boobs ?

You most likely have a heat rash from wearing a bra for two days.  You didn't let the skin "breath".  You didn't do anything to get the sweaty material off your skin.  (and - as already mentioned - if this was NEW bra and you didn't give it a pre-wash in your own detergent - you might be having an allergic reaction to the material because of any chemicals or detergents that may have been used on it when being manufactured.)  

Wash the area and keep it dry.  (baby powder in the area may help)  Avoid wearing anything too tight.  Avoid wearing a bra of any kind for more than about 12 to 16 hours.  Benadryl can help with allergic reactions if it is allergy related.  If it is just a heat rash - it should go away in 24 to 48 hours.

It sounds like you may be allergic to any chemicals or dyes from the bra.  Toss it.  For the rash, apply the gel from an Aloe Vera plant.  Make sure it's only from the plant.  

It sounds like heat rash. Wash and dry it thoroughly and then keep it is as dry as possible (baby powder or corn starch). 

Had you washed the bra before wearing it? Fabrics are often full of chemicals that can irritate the skin. Clothing, particularly anything that fits firmly, should be washed before wear.

It could be as simple as the fabric causing moisture build up resulting in irritation.

1.  Are you one of those people who don't wash new clothing before wearing it?  New clothes are full of fresh new chemicals that should be washed off before wearing.

2.  It's from sweat...wearing it for 2 days without washing it is asking for trouble.

3.  Are you allergic to the material?

4.  The rash could also be caused by it being too tight and misfitting.  I always hear stories of ladies buying bras and not liking them because they aren't wearing the right bra or wearing it the correct way.  The recommendation always seems to be to get them professionally fitted at the store.