What is the small, swollen, lump on the calf's knee?

I noticed the calf limping a bit last night, this morning she had a bit of trouble standing (a VERY tiny bit of trouble). Felt her legs for heat and swelling, noticed a fluid filled lump, quarter sized, on the left knee. The skin seems a flaky at the site, it's very painful, she wouldn't let me touch it. What could this be?

Any chance it has a thorn in there?

Wound of some type, maybe a snake bite, or a bruise?

I'm concerned about "joint I'll". The navel area has caused enough concern The last few days that I've been keeping a close eye on it. The cord was dried and clean until it fell off. Then, I've noticed a bit of crusty hair around the navel, I've been cleaning it and packing it with bag balm to keep the bugs out. But I've read about navel I'll/ joint ill and now am concerned about that. What should I be looking for as symptoms of that?

A small swelling in the knee would make me think joint ill. All joint ill I've seen the whole knee was swollen and geenrally they don;t want to put any weight on that leg.

That's what I'm thinking, however the lump appears to be directly under the skin, and outside the joint- if that makes sense. It's a distinct bump, and not the entire joint. The vet is coming this afternoon to take a look, so I will update this evening.

My guesses have already been taken snake bite or thorn. Hope it ain't nothing serious.

Any news yet?