What Causes Pimples On Vulva, Stomach, Arms And Legs During Periods?

hello, although i havent had anything that looks like this, around the time i am starting to get my period, i keep getting these pimples on the inside of my vulva, sometimes on my stomach, and on my arms and legs, also over the summer , i had a bad rash break out on my butt and lower back, i have been, and i always test for hiv, hepatitis, std s, and the like my test always come back negative, please further advise, i am concerned