I've been having really bad pains on the left side of my stomach, what can this be?

Several things: Problems like these can only be correctly handled by your doctor in person. Left-sided pain can denote anything from constipation to colon cancer. It's best to be checked.

Anything: Most people think their whole abdomen is their stomach, but the stomach is in the upper middle or upper left of the abdomen. If your lower abdomen hurts, that could be your skin, muscles, bladder, uterus, or intestines. If the upper part of your abdomen hurts, it could be acid reflux, pancreatitis, or other problems. See a doctor for an assessment.