You should go see your primary care doctor
about this who will be able to help you figure out what is going on. I can think of a few possibilities.
First, it could be a cyst under the skin. Cysts can develop anywhere on the body under the skin. They are generally benign, but sometimes they need to be surgically removed because they cause pain or because the continue to grow and are unsightly.
Second, it could be a lymph node. There are lymph nodes in the crook of the elbow (the inner side) that can become inflamed or swollen in response to various infections in the fingers, hand, or wrist. If you recently had a cut on your left hand, that might explain what is going on.
Third, it could be bursitis. A bursa is a fluid filled sac that cushions a joint, and there is a large bursa in the elbow that frequently becomes inflamed. This may be tender or not, but usually the swollen area will be 'squishy' and feel like it is full of liquid.
Your primary care doctor can take a look at the elbow and consider these possibilities as well as others; they can also recommend a treatment, if one is necessary.