Mastitis caused by MRSA

I just found out after an er visit yesterday that I have mastitis caused by MRSA. I am so confused how this would have happened or what caused it. I am an exclusive pumper and I didn’t have any cut on my breast or nipple. I was originally on antibiotics for mastitis but wasn’t getting better. Now I am on a super intense antibiotic and I have to dump my breast milk. Has anyone else experienced this and can share some insight?