I found a hard lump on my 8 1/2 - 9 year old dog's nipple..?

I noticed it when my puppy went up and sniffed it. I went up and felt it and its hard. /: She's a boxer mix and she's either eight and a half or nine years old. She seems happy and healthy, but I'm just not sure if it's a fatty tumour, which I heard was usually soft, or if it's cancer. /: