In case you are experiencing severe pain in sitting even for 5-10 minutes, I would suggest that you undergo an X-ray of your knee. With the X Ray, we can get a fair idea of what exactly is wrong with your leg and what is the reason behind the pain
Generally athletes get ligament tear, in common words sprain due to excessive rotational force which will cause pain if not healed properly. Since you can able to walk and withstand the weight of your body normally from past one month chances of having fracture is less, pain may be due to ligament injury which would have happened while you are playing football, generally it takes 6-8 weeks time to heal but if you resume your athletic activities in between without taking rest, you will have pain again. So consult a orhopedician for further management.