Why are my dogs swollen hard and bruised.?

i have a chahuahua mixed with a weener dog. Her back 2 are hard and swollen and alsobruised witha bluish purple color. She hasnt been spayed. and i read somewhere online it can be cancer. I am really worried about herr. also they dont seem to be bothering her because when i felt them to see if they were tender and sore she didnt show any pain. she is not acting any different. Thanks for the help.