What does a pimple inside the mouth followed by swelling in the neck and jaw mean?

Infection: Dental infection or abscesses have points to which it drains. Looks like a pimple. The point of drainage is a fistula. When a tooth becomes abscessed or a cyst or other pathology drains it will follow this path of least resistance to the fistula. If the infection is significant, jaw swelling or lymph nodes near or in neck drain fluids as well and this is consistant with swelling described by you.

Infection!: The pimple in your mouth could be from an abcessed tooth--the pus trying to escape through your gums. The swelling in your jaw and neck could also be from a tooth abcess--the infection spreading from the bone into the soft tissues. See a dentist asap! if you have an abcessed tooth causing problems like you have described, you could get very sick very soon. Get treatment.