Alright, so my sister noticed a hen walking abnormally. Pretty slow and sick-ish looking. She also seems to be missing her tail, under which a medium sized swollen lump has appeared on the top of the vent. It's not solid and can be squished, but I didn't want to poke around and harm her, and there seems to be a sort of liquid coming from her vent. Pictures will be below, however, the quality probably isn't the best. Answers needed.