Nipples inverting?

I was rubbing Pineapple's belly today and i thought she had an absyss or something so i took a closer look and it turned out to be her nipple, it looks like it is "caving in" on itself, starting at the very tip. I had a feel around and it is very lumpy underneath it. I also noticed that there was a bit of gunk sitting in the "crater" at the tip of the nipple, i squeezed it very gently and it came out, much like when you sqeeze a blackhead or pimple. I looked at the other nipples and noticed that it was both of the top nipples (the ones just next to her front legs) that is doing it. It looks very pinky/red, very swollen and there are lumps underneath and around the nipple. Does anyone know what could of caused this? She was on antibiotics last week and recently had her leg amputated but appart from that i can't think of anything that would do this.

ok, I looked at them again and it appears that the very top two look like the inside of the nipple is coming out and the outside layer is sliding down.
Then the second pair down from that is the infected ones with stuff coming out of them.

I have never seen anything like that but it doesn't sound good. I would have a vet look at it right away.