Did i sprain something in my back or is it my kidneys??? Help! Please?

Hello. Im a 23 yr old female, healthy and active. i woke up Saturday morning with my back feeling a little off. Not too bad though. But it was very uncomfortable to walk. Lower back, left side, not right where my kidneys are but lower above the hip. I thought maybe it was from wearing heels, i always wear them so i dont know. So the next day on Sunday (yesterday) it was way better, i didnt feel a thing, BUT i did wake up feeling achy all over, my body, my legs, my arms, my thighs, even the muscles in my tummy, like my abs, almost as if i went to wrk out the night before which i didnt, i felt all sore. Dont know if i was getting a fever or not, So sunday night last night the aches pretty much were gone, but the lower back discomfort started coming back. I went to sleep, this morning i woke up fine, i work in a office sitting in a cubicle all day, ive never had back problems before in my life. But since this morning its getting worse. Its a sharp shooting pain, almost like someone is pressing down or against my back, feels like the pain is radiating down to my ocks even down my thighs at times. sometimes i feel okay, the discomfort is still there but all of a sudden sharp shooting pain on my left side lower back. I have no other symptoms. No fever, no aches, no chills. So is this a sprain or broken muscle of some sort or a kidney stone, or kidney infection? Im going to my doctor today once i get off work. I cant wait i cannot stand it! Why is it that it went away Sunday and just came back, wouldnt a sprain or broken /tightened muscle stay there? the pain wouldnt just go away and re-appear? This is why im thinkin that its maybe something to do with a kidney stone or infection cause the pain went and came back. BUT its NOT in the KIDNEY AREA. Hurts to walk, and even just sit here. Feels like theres a huge boulder sitting inside my lower back! feel all tensed up. Please someone what could this be?