What Causes Sore Inside Lower Lip?

I have had cold sores before as a child but not recently(within the past 5 years). Last night my lips were very chapped and i was biting on them. A nervous habit. This morning I have the beginnings of a sore inside my lower lip appearing, no redness or itching. It has dark spots on the outside but no blisters. Is it just a cold sore or should I get it looked at for a possible STI?