can be a lot of things... where exactly on the stomach is it? Is it soft or hard? Does it move with the skin or does it seem to be rooted deeper? How old is your dog?
Small umbilical hernia is one (center of belly, usually not a problem), fatty lipomas (very common in older dogs and generally harmless, soft lumps that move with the skin), mammary tumors (hard, located near a teat) old age "warts", bug bite...and on and on
Keep an eye on it for any changes and mention it to your vet next time you go. However if it does change in size or texture you will want to see the vet about it