Am I getting an allergic reaction?

A few days ago a small area began to surface under my right nipple. It was about an inch long and it looked something like a mosquito bite and it even itches sometimes. Today whatever this is is spreading out causing more bumps resembling bites but i know that is not what it could be. I wonder if it is a rash of some time or possibly an allergic reaction. My girlfriend suggested that it might be the latter do to my change in washing powder. I had been using Fab before but now I am using Trend. I am hoping somebody can provide me with some answers.