Gallbladder is right upper quadrant, so that's probably not it, although sometimes pain can get referred to other areas of the body which makes diagnosis difficult. Typical gallbladder pain is right upper quadrant - right under the rib cage - and often includes right shoulder and shoulder blade pain.
beebee - is your pain on the entire left side or in the upper or lower left quadrant? Any intense pain means something is wrong, and you should be evaluated by your doctor. Since it always happens at night, my guess is that it's your intestines. Start with your primary care physician. If he can't figure it out, you'll be referred to a specialist like a gastroenterologist if he thinks the problem is with a digestive organ. If the pain is extremely low in your pelvis, then you might check with your gynecologist.
Isn't that the side that your gall bladder is on? You might want to get that checked, one of my co-workers had to had surgery to get hers removed and if I'm not mistaken I think she told me that the pain travel to both sides at times. Look up gall bladder to find some answers maybe and hope you feel better soon!