Every now and then, my left nipple becomes really itchy, this happened a couple of days ago, and now I've been getting dull pain all around the breast. This breast is smaller than the right. I'm only 18, there is no way I can be pregnant, and I'm not due for my period. Is this just a sign that my left breast is catching up to my right? Or could it be something much more serious?

Most likely it means you have a touch of thrush or other minor infection in your left breast. Nearly all women have different sized breasts, it's nothing to worry about, but it's unlikely that one will catch up with the other (unless you are breastfeeding from the left more than the right, and even that is temporary)

If it doesn't go away fairly quickly, go to a doctor and get it checked out, so you can get the right treatment.