Hello!
Is the bump soft like it contains liquid or firm?
Size?
If it is about the size of a golf ball or slightly bigger + soft, could be hematoma, caused by a break in the blood vessel like a bruise. It is very common to find them in between the front legs, my horses have gotten them a few times. Caused by another horse kicking, running into a fence, stob etc...They generally went away within a few days but if it doesnt, If you know it's hematoma you can shave a small spot on the middle of it and lance it with a sterilized needle. The needle will probably run with blood. You want to make sure that it has stopped filling with blood, a day or two after it has happened usually.
If it is firm and doesn't feel inflamed, could be some type of tumor, but I would get a vet's opinion on it if it is hard.
Either way though it's really best if you could get a vet's opinion also...

Hope this helps ~