Sore, red, blistered, slightly swollen lips with no previous allergies. what could it be?

Blistered lips: How long have you had this condition. One possibility is a herpetic viral infection (cold virus). This usually is self-limiting and runs it's course in 7-10 days. If you have had this for longer than 10 days, you should check with your physician to make sure this is not part of a systemic infection or disease.

A few possibilities.: It may be an allergy that just developed. It could be over indulgence of citrus fruits, particularly grapefruit. It can be a viral infection (such as herpes simplex i. The best advice i can give you is to see a physician if it persists more than a day or two.