So this might be sometimes confusing: how can you tell the difference between a cold sore and a pimple. Well, when you have something and not sure it's a cold sore or just a normal pimple then you may try this yourself diagnose.
Get a hot towel and wring it out and wipe it across the skirmish aggressively and if white stuff comes out it a cold sore. A simple, regular or big, will nevertheless be a single bulge. A cold sore will most likely be a bunch of smaller blisters in a place of redness. They may both itch, burn or sting, but the cold sore may also be numb.
Hydrogen peroxide: You can find a little bottle at any store for approximately $1.00. I have always found that it works best for any kind of sore in the mouth. Pour a small amount into a cup to make it easier to pour into your mouth.
Swish with it, do not gargle and do not swallow. Just swish. Use it full strength, it is going to have very little flavor, almost like water (but do not confuse it with water) and it will sting and burn off a bit while swishing.
This is perfectly natural. Spit it out the second it gets too much to bear. It'll get foamy, that is normal; it just means that it is doing its job. Rinse out your mouth with water a few times and wait for a little while, then repeat one more time with the hydrogen peroxide and swish a few more times with the water.