My dog had something on her belly so I took her to the vet. While what I took her in for had disappeared, the vet said she has mammary tumors (breast cancer). They may be benign, but the only thing that can be done is an invasive procedure to remove everything.
She's fine so I'll just keep an eye on her & do follow-ups with the vet.
I didn't have my dog spayed since I knew that there'd be no way she'd be able to get pregnant. I didn't know of side effects though. Hopefully this helps someone.