Don't want to worry you at all, but best to get it checked out.
My lame horse was seen by the vet who couldn't find anything on her portable scanner, so referred us to York
He had been a little lame, so i rested him, then he pulled away from me when leading him on the yard, cantered up the track, jumped the fence and proceeded to gallop round the field with the others.
By the time we got to go to York, he had a swollen Fetlock.
When the scaned him , they said he had done awful damage to his deep digital flexor tendon. So bad that his had ruptured and burst from the sheath. He likened it to having a swollen foot, but still wearing your shoe, so your foot had nowhere to expand to, therefore was very painful.
From then on we trod a very long and slow road to recovery. They told me i would probably never ride him again. I threw every penny at him i had, to help him get better, supplements, lasers etc.
And better he got!!
So this could be an awful injury, but just as easily could be a twist or sprain.
Get him checked out as soon as
Could be an abcess brewing. Our old brood mare got regular abcesses (long story) but we always knew when one was festering up as her fetlock started to swell up. She had remarkably hard hooves and almost every time, the abscess burst from her coronet.