My fourth toe (on right foot only) became swollen with no trauma 3 days ago. swelling now spread to foot/ankle. no bug bite i'm aware of.

Infection : Infection is a possibility. You should se a physician and get a definitive diagnosis

Be seen: I would definitely be seen. The concern would be if you have some type of infectious process that is spreading up your foot. An exam and lab work would go a long way in making the diagnosis.

Cellulitis.: It is obvious from your description that something is definitely amiss. There does not have to be a wound in order for the toe to become infected. Cellulitis is definitely a possibility. Consult with a physician quickly.