OH! My gelding had the same thing when he was in training years ago. I went up to see him on the weekend and found a very similar thing on him (not quite as big). It wasn't hot and didn't seem to bother him. Went down over time, but the vet finally had me do DMSO to finish off the swelling. Trainer had no idea how he could have done it. Definitely at least put a call in to your vet in the morning.
Is it squishy or hard? If it's squishy it's most likely a seroma/ hematoma (although it could be an abscess and it's still a good idea to check it.) If it's hard I'd be more concerned. I wouldn't assume it's related to something that happened 6 years ago. It looks like trauma to me.