My grandmothers cat has a tumor?

Hello everyone. My grandmas has a older cat with a rather large tumor under her nipple and it is making it enlarged. We are getting ready to book a vet visit for this. But I was trimming up her nails earlier (she is very mean, and always has been) and I believe she got scared and it seemed like her nipple started leaking... No blood but a clear liquid. I know this sounds disgusting but I was wondering what that was and what could have caused it. I looked it up online and couldn't find any answers, does anyone know what could be causing this?