hi ! acne sort of thing that which you have might be nipple wound, which are common in females ( very few males also). tight clothing, or unknown scratches, due to bras etc lead to such wounds,and more over you are in period. need not worry about it. consult a gynaecologist and get checked once , if needed got for mild medication, mostly external application can heal those, but if they are often you may have to be treated for them properly.
Health Tips
consult a good homoeopath and get treated, so that you may not get those painful symptoms during periods. menses is natural healthy process for women, not a painful crisis.