Lump on cat's stomach

Hi I have 3 cats, 1 is 3-4 y/o female not fixed, had 1 litter of kittens
about a year ago, 2nd one is a 2 y/o male fixed and the other is a 8-10 wk
old female kitten. Since the 2 adult cats have gotten used to the kitten
being here and actually like her, the kitten seems to like nursing on the
adult female. Has been going on for 2-3 weeks. Well now on the adult
female's stomach in a big patch under 2-3 nipples there is a hard lump there
that is painful to her to the touch. This has just developed in the last few
days. Any suggestions of 1: how to get the kitten to stop nursing and 2:
should I worry about the lump or just get the kitten to quit nursing and
it'll take care of itself?

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