What Causes Small Bumps Under Skin Of Thumb?

Recently, I have been getting these small bumps under the skin of my thumb that appear and disappear. Also, there is a very distinct almost perfectly square patch of skin that is very dry and cracked on the pad of my thumb. Any ideas?<I am an 18 year old female, 6'0 tall, and 160 lbs. I have no pre-existing skin conditions.