My left soft palate is really sore and swollen. it only hurts on my left side of mouth and has been like this for 3 days. what could this be?

Probably a cold sore: You may have a canker sore caused by the herpes virus. These are especially painful little lesions that may occur in the mouth. They may be so painful as to cause it to be difficult to swallow! you could try some Valtrex (valacyclovir) from you fp to decrease the pain intensity and duration. For the most part, this is a viral infection that may just take some time to heal. Continue with supportive measures.

Virus : You probably have some viral infection. Common to attack one side of head. It can effect the whole side of head, ears, throat, etc. You may have some lesions that you cannot see. You need to be seen by dentist or physician to properly diagnosis and treat problem.

Could be an abcessed: Abcessed teeth will often cause pain on the palatal side of the tooth.