During sex and during ejaculation, I have pain in my left groin area. It is like pulling of the muscle. Could it be hernia?

Could be. Hernias are usually associated with a lump, but in the early stages may not be. See your primary care or a general surgeon for a physical to check it out if it persists or gets worse.
Groin Pain. A hernia occurs when the tough connective tissue that completely covers our bodies (under the fatty layer) develops a weakness or true hole and the stuff inside bulges out. If you are having pain, it means that the area had some injury. If there is a bulging and it does not reduce easily, then you need to see your doctor. Otherwise, Motrin or advil (ibuprofen) should help reduce the pain.