Swollen hind Fetlock?

I just bought an OTTB a few weeks ago, he hadn't been ridden consistently for about a year so I brought him back slowly. We started doing moderate work a few days ago i.e wtc, jumping small cross rails, not riding any longer than 45 minutes (most of the time just 30), but the ring is kind of hard. Yesterday when I went to go ride I could tell he was off, and noticed he had some swelling/heat in his right hind fetlock (a little in the left too, but mostly the right). Today I cold hosed both and put liniment thats supposed to help reduce swelling on them too. He is pretty stiff, what do you think this is? I thought maybe it was just windpuffs but after reading I saw that they don't normally cause lameness or heat.