Mysterious Fetlock Lump?

I brought my mare in from the paddock and when I went to pick her feet I found she had grown a large lump on the outside of her left fetlock. It wasn't there yesterday when I picked her feet after I rode her. She wasn't lame, hot, etc. What could this be, as of now she is in a private paddock as she is getting over a cough. So that rules out a kick or a bite. Could a bad bug bite cause such a large lump(about 3cm sticking out and 5cm long)? She wasn't lame, but she didn't like it when I touched it. I don't know she has me stumped once again!
Thanks for any advice :)

The fetlock joint makes me think of windpuffs. But if she is just getting over a cough, I would imagine you haven't done any hard riding that would cause a lot of concussion, so windpuffs would not make complete sense. Is the lump squishy or hard? Is it hot to the touch?

It is very squishy, I had ridden her harder yesterday, as she had no signs of any cough, we did a little Dressage but nothing more than a few half passes. I am not aware of windpuffs...What is it exactly?

A bee or wasp sting could easily make a swelling like that. Perhaps a ground wasp?

Wind Puffs Are Swelling in Your Horse’s Ankle

This link might help. I don't know enough about your horses injury to say I think this is what it is, and I am certainly no vet, but check it out.