Newly purchased cow or one you've had a while? Had a bull that was favoring his right hind leg, figured due to fighting, but segued into a full limp. I could see & feel some swelling but wasn't sure what was wrong so took him to the vet. Vet shaved his leg all around the hock & fetlock and discovered a small scar from a previous wound, looked like a puncture. Opened up the leg around the fetlock and the amount of pus that came out was unbelievable. Ended up putting in a rubber tube so the wound/pus would drain, I flushed it every day for 10 days and pumped him full of penicillin, completely healed.
That said, if you put a healthy cow on a trailer & she comes out limping is it possible you drive like a maniac? ;-)