Hello~Normally, if it is a blood clot, it will hurt constantly and even more so when walking or exercising. I tend to believe that it is a pulled muscle, or even a misplaced vertebrae in your mid back. I would like to suggest a visit to a chiropractor, he/she will take a spinal x-ray and be able to determine where the trouble is, a few adjustments should have you pain free in no time.
Hello!
Blood clot pain tends to be constant and not relieved until the blood clot moves or dissolves. Additionally, there'd more than likely be noticeable swelling in one leg and not the other, and that leg would be hot to the touch and the other would not be. I speak in general. It is important to seek medical help from a professional, if you are concerned about this, as each case is different and can present different - even unique - symptoms.
I hope you feel better soon!