Injury Guinea pig sore and black nipple

Not sure how old our guinea pig is. I only know that we have had her for a couple of months. She was well taken care of before we got her and is still happy and healthy. She has gotten use to us and I even taught her some new things. Well just a while ago, I had just got home and I took her out for some lap time. I was petting her in her fav. Places. Under the chin and down her bum when she jumped and squeaked a little. That's very unlike her. I felt around her back area until she jumped again. I don't know what it is. It could be her back leg or her butt or her nipple. I turned her over and saw that she had a black nipple. I don't usually feel or look at her nipples because she's very sensitive down there so I don't remember if that's always been there or not. I read that a black nipple is normal and all but why is she so jumpy when I touch around her leg and butt? She's never not liked me petting her around that area but she sounded like she was in pain. Any ideas on what this could be? Any way to treat or fix it? Oh and she is the only guinea pig we have.