This is a general low back ache and for this you can follow these measures: one keep a pillow right under the knee while sleeping, next is you can keep ice in the painful area for about 5--10 minutes, if pain
still persists you can stretch your body by twisting the waist on both sides how we used to do in the school drill similarly you can try which will help you relax as well will reduce the pain. It looks like you are anaemic. If you have back pain
after you sit for long hours then it is due to your haemoglobin levels as it is important to check that. Anaemia always leads to the symptoms of being tired and also having back / leg pain
though there may not be any pathological reasons for back pain. If you have leg pain then you have to rule out the casues for having leg pain. First of all check your weight and your haemoglobin levels, as Anaemia always leads to the symptoms of being tired and also having leg pain though there are no issues with the knee joint or back pain. If not if the pain radiates down the back of thigh and legs then it might be due to sciatica
. Kindly consult the nearby physiotherapist
. Hope you recover soon from the leg pain.