You need to make an appointment to see your doctor for him to arrange to send you for some imaging tests (xrays the very least or an MRI scan).
It does sound very much like you have a sciatic problem. When you look in the mirror, the reason for the left side being bigger could be due to inflammation.
Once you get those done, the doctor will be able to find out what the cause of your problem is to be able to give you the correct advice regarding resolving the matter or at least managing the problem.
Also ask your doctor to refer you for physiotherapy.
In the meantime apply heat to the area. You can purchase different heat packs, or use a hot water bottle. You can also purchase anti-inflammatory creams that you can apply and see if that helps. If you find the heat makes it worse, then try a cold compress.
Make an appointment to see your doctor and get checked out.