I felt some pain in my knee after playing soem football. I am able to walk around but I limp and the knee is not swollen. Is this a serious injury?

Varies . It varies somewhat, based upon the specific cause. It might be reasonable to ice it down and rest it, if it does resolved over 3-5 days it probably was a minor injury. If it does not get better, or worsens at any point, I would not hesitate to be seen for it.