Acne-like irritation at the corner of my mouth

I went on a two week vacation and on the long car-ride home I began to feel something similar to acne developing under the skin at the left-hand corner of my mouth. I didn't think much about it until the next night when it had become red and painful to open my mouth. It looks like a pimple, but in the little acne I've suffered from it's never been that close to my mouth. It's been 3 days and since gotten a little smaller but not much and a less firey red. Thankfully it's not too noticable regardless of the minor swelling of the area. At first I thought it was acne,but I don't much, then a cold sore, which I've only had one of in my 16 years, and now I think it may be some kind of infection as acne treatment isn't working. I'm not too concerned, I just want it gone!!! Any help would be appreiciated.