For about a week now I have had a red lump on the side of my left breast. It hurts when I touch it. What could it be?

Red breast lump. It could be an infected lymph node, and infection mammary duct or cysts, an insect bite, or an inflamed blocked pore. It the area were large, there might be an underlying abscess. If it is large and solid, you always have to think of the possibility of cancer.