What do you think about my horses condition?

this morning we found my horse with what seems to be a swollen vain or possibly tendon on the left side of his chest and running down his left leg. This was causing him pain and he was unable to move freely. The local vet has been very busy today so he hasn't been seen to but we have spoken twice. there has been much improvement through the day and is eating and drinking. he has not done anything strenuous recently that we know of. he is a strong 7 year old TB gelding. has anyone heard of this condition.