Cheek swelling is most commonly caused by acute bacterial infection. Spreading of the infection must be avoided. If you are addditionally feeling nauseous and/or feverish, immediately consult a dentist.
Please note that swelling virtually never heals itself. You can cool with a cold damp cloth and avoid physical exertion and exposure to sunlight and heat.
Other causes for cheek swelling can be muscle tensions, inflamed salivary glands, etc. Always consult a dentist.