My 5 Year old Baby Girl Lump on one of her tittes

Please Help me.  We have A 5 yr old Pittbull Dalmation, She is a rescue dog that was used as a puppy mill when we found her. She Had many many litters of pups her tittes never went back to Normal and at the time the Vet said she had as well. Recently I noticed she has a cyst or nodule on when of her Nipples, it doesn't seem to hurt her and I can grab it in my fingers and move it a around Like a marble. She is not fixed yet but we are going to be doing that in Feb we live on SSI and Money is so tight. This little girl means the world to My a husband and I She is the most Loving Dog I have ever seen. She is not sick shows no signs of being sick, but I am scared to Dead as what this could be. Please advice and maybe give me some ideas of Vets that are not so expensive. She will be going into another heat in a week or so and we just have no income until Feb. Thank You for your advice as I am worried sick.