If it's still there tomorrow, she's not uncomfortable and your careful not to get kicked, press your thumb into it to check if a depression is left behind. If there is it indicates that there could be fluid under the skin which is commonly called edema(sp). Usually it's found along the belly or in the legs as the fluid naturally finds the lowest point. If it is harder then that it may be a bite
What causes it IF that is a edema?
It may be a irritation or infection somewhere in the horses body area, or an allergy to feed or something, or even a sign of a sand burden or a virus, so keep a close eye on it and if you are worried because there's no improvement don't hesitate to call the vet.
We have a 32yo horse here with a permanent edema that can not be pin pointed unless he gets a belly tap (A procedure where a biopsy needle is inserted into the belly to see if there is a problem with the digestive system of the horse, but the owners opted not to do this due to age and the quite high risk of further infection after talking at length with the vet) The horse has been fine like this for more then six months with no improvement or deterioration.
It may be an impact injury (in which case it may be a hematoma which is blood filled and if it continues to fill with blood and get bigger, it may need to be drained or it could harden and become permanent)
Either way if it was my horse and I was worried, I would think a vet is the best idea.