What Does Recurred Severe Pain In Jaw Suggest?

At first i have a pain at my left jaw, I can't open my mouth and it hurts a lot, the doctor said I was just a muscle pain at my cheek so he gave me a medicine to relax my muscle (sorry I forgot the name of the medicine). And then the pain was gone. After a few months the pain was back again but the pain is not as bad as the first time and I only have it when I open my mouth wide and chewing hard.
Is my condition serious? Can the jaw exercises can help? and Should I see a doctor again?
And also I sleep facing down and sometimes with a very low pillow. Is it possible that I got it in my sleep position?
I also have 3 molar tooth remove. But I always go to the dentist and I think I have no problem with that.
Thank you very much.