I have had this same problem and then I had this same problem in my tummy. As soon as I got it on my tummy I went to my doctor and he said not to worry it will go away. It did go away for me. BUT it is good to see your Doctor.
There are so many causes for itching and rash that you should see your doctor to try to figure it out. In the mean time, get some over the counter hydrocortisone cream, which will help the itch and reduce the rash. You can also take benedryl at night (to avoid the sleepiness bothering you), which I've found to be helpful with itchy bug bites.
If the itching is under your breasts, particularly if you have larger ones, it' can be a couple of things. Sometimes it is a build up of heat which results in a heat rash or if you sweat a lot, the constant moisture causes the itching and rash. I have found that cortisone creams do not work, but amazingly diaper rash cream does the trick. Of course you should consult your doctor to find out for sure.
Yes, absolutely see your doctor. There are as stated above many reasons for itching breasts. Some due to skin and some due to other reasons. Important to get diagnosed properly. good luck dear