Sounds to me like Sacro-Iliac joint pain. If traumatic usually caused by a fall. Initial starts with pain in the back, but soon after the surrounding muscles begin to spasm/tighten to try and protect the injured area.

Best thing for you to do is put ice the the injured joint (normally it is below trouser waistline, a couple of cm away from the spine). The trick with this problem is not to do the same thing for long periods i.e. sit for a period, but as it starts to ache, walk around until it starts to ache and keep doing this. Main thing is get to see a physio/sports rehabilitator asap as it sounds like it is beyond your own help.

Last thing to try, (I will try and explain). Lie on your back on the floor (as long as you can get back up), bend both knees so that your feet are flat on the floor. Then put the ankle of your sore leg onto the knee of the other leg. Press down on your knee until you feel a stretch. If you don't feel anything, then use your hands to bring your non-problematic leg towards your chest. Make sure you don't put yourself in pain as this will just cause everything to tighten further. DIscomfort is ok though.

Hope this helps