I have a pimple like bump inside my belly button. I had one on the outside also. It is red and leaks pus that has a foul smell. Is it serious?

Umbilical infection. Previously answered, but brieflly--infected material, needs to be removed and drained and possibly need antibiotic--see your primary doc or general surgeon.
Belly-button-buildup. If you have an 'innie', then skin epithelial, dirt particles, skin bacteria and other small material can build up in the umbilicus. When it builds to a certain point, it may become infectd and forms a pocket if infectd material that becomes bloody pus. The foul smell relates to the particular bacterial infection; must c gp or general surgeon to have it draine and removed; then clean out regularly.
You have infection. Some time if this kind infection is not treated properly could lead to a serious infection.
Patent urachus? There is a duct from the bladder to the umbilicus (belly button) called the urachus that is present in fetuses. It usually closes after birth, but sometimes not. If this is what you have, a small surgery can fix it. See a general surgeon or urologist.