The back of my knee hurts really bad after walking what can cause this? and is this a symptom of a blood clot?

Pain behind the knee has many causes. As simple as tight gastrocnemius muscle or even a tight hamstring muscle. Both muscles cross the joint. Without extremity swelling clot is unlikely. However, you should still be evaluated and treated. A physical therapy referral is probably indicated or see a sports medicine podiatrist and be instructed how to develop a running program.

It is not feasible to provide a meaningful opinion without additional history, physical examination and may be some tests. It is unlikely to be a blood clot. You may be developing a Baker's Cyst. For information: https://www.mayoclinic.org/diseases-conditions/bakers-cyst/symptoms-causes/syc-20369950#:~:text=A%20Baker%20cyst%20is%20a,straightening%20or%20bending%20the%20knee.