Skin on big toe filling with pus and swelling. what is happening?

Infection: You sound to have infection on the big toe. Are you diabetic? Do you have health problem? Check it out.

Infection: Have it evaluated right away. You need to have this treated to avoid preventable complications.

Possible ingrown toe: Nail if by the nail but obvious infection present. See podiatrist sounds like drainage will be necessary and possibly antibiotic.

If there is pus...: It already is infected. It needs to be drained and cleaned for the infection to go away. If it spreads into the deeper tissue, you will need to take an antibiotic to get rid of it. Use warm water soaks with some epsom salts, dish soap, or even a splash of vinegar (most bacteria aren't that tolerant of an acid environment). Keep the pus draining out of the hole that's already there.