My horse is an Irish cob and we do a lot with him, including showing, dressage, show jumping, eventing etc.
I was riding him last thursday practicing for a dressage competition and he didn't feel 100%. I ignored it and carried on but it got worse as I was riding him. I got off him and hosed him off, and cold hosed the leg for about 15 minutes. I put him into the stable and within 30 minutes he wouldn't stand on it and refused to walk. We called the vet out and he said it was a fetlock infection. He gave him a shot of penicillin and he now will stand on it but still limps while walking, and it is still swallon and sore.
I noticed while I was hosing him earlier that he had 2 red dots, about 1cm big, exactly the same in the same place on each hind fetlock joint. The swelling has gotten worse since the vet came out and he now doesn't look sound on both rear legs. The dots are red and swallon, but are in the exact same place above his fetlock, not on the actual joint. I have given him bute and cold hosed the area but he still isn't happy with it.
I think it could be more than a fetlock infection but I'm not sure. Any idea on what it could be and how we can treat it?

Of coarse without actually seeing the horse it sounds like cellulitis (a diffuse bacterial infection of the skin and associated tissues)

Although if it is than the joint fluid sample taken may have been comprimized & not be able to get an accurate reading, you may want to consider using ultrasound to image the joint.

Also, slightly confused as to why only 1 treatment of penicillin.
What was your follow-up care suppose to be.

Hopefully it is something minor.

I would ask your vet, but could be something minor like scratches?