Why does my cat have itchy lumps on her belly, but it's not a tumor?

Thank you for submitting your question regarding XiaoMi. It appears that she has a skin infection on her abdomen. Mammary tumors grow out of the mammary gland under the skin. The exception is if the tumor becomes large enough, the overlying skin can ulcerate and rupture. This does not appear to be the case though from the attached pictures. I do think that she should see her veterinarian. She will likely need antibiotics and a medication to control any itching. She will likely have to wear an E-collar to prevent self trauma as well. If the cause of the infection (if that is what it is) can be determined, this can help to prevent future recurrences. Allergies are common ok the abdomen. I hope this information helps!