Most probably you are suffering of arthritis
. Please understand that the arthritis pain is flunctuating: some days, it is high and others less.
In addition, have you had any accident causing injury
to your left leg (at the area you are having pain)?
If this is the case, talk to your doctor to perform an X-ray to exclude any possible problem related to that accident and follow the right treatment.
If you do not have any accident, most probably is because of arthritis. Being physically active, anti-inflammatory and analgesic
drugs and some changes in your lifestyle and diet can help you ease such problems and reduce the frequency of the pain.
You can discuss all these with your doctor to get the best therapy for you at your local place.
Hope it helped!