Can you get blood clots from just sitting for a long time? i'm 21 and i workout regularly but lately i've been sitting for long periods of time and my knees and the bottom of my feet started to hurt.

Yes it is possible: It is possible but unlikely. You probably are compressing the nerve behind your knees and this is creating the pain.

Unlikely: It's pretty unlikely as you will develop a DVT (deep venous thrombosis) from just sitting....Especially at your age. However, some people do have hereditary clotting disorders, in which case it's more of a possibility. If your family does not have a history of dvts or pes, you have an extremely low chance of developing a problem.

Yes: Prolonged immobility increases ones risk for developing blood clots. Prolonged plane rides, car rides, etc can lead to clots. Staying well hydrated and getting up and moving around periodically reduces your chance of developing blood clots. While sitting for prolonged time move you feet and legs can also help reduce your risk.

So to answer your : Question definately, prolonged sitting is one of the risk factors of getting a clot. You also state you have knee and foot pain. There are other reasons you can have knee and foot pain though - do you have flatfeet? If so you can have plantar fascitis and flatfeet also put pressure on the inner knee....So since you mention a clot - that should be ruled out as that is dangerous, other causes too.