I have experienced what sounds like the same thing. Can not really determine whether its my hip, low back, or in the butt muscles or hamstrings. It started about 2 weeks after I started doing Atkins (mid Jan.) and got tho the point where I went to the doc about it last week. Had an x-ray and it shows nothing. Having a bone scan of the area on Monday. I am now taking 800 mg of Ibuprofin three times a day. It only bothers me when I get out of bed and for about 1/2 mile into my walk. Tightens up again in the evening.