I jammed my thumb playing hockey. i've kept it immobilized for 2 weeks. i can flex it, but if i put pressure on it when i extend it, i have sharp pain?

You may have an....: Avulsion fracture where the tendon has pulled off a piece of bone where it attaches. You should see a doctor & probably get an x-ray.

Ligament injury or: Laxity is acute or chronic. A collateral ligament injury can result from a hyperextension injury on either the radial or ulnar side, and this is often repaired surgically if comlpetley ruptured. Unrepaired chronic injury leads to arthritis in the joint below in the base of the thumb and can be addressed if needed . The thumb is complex and a lot of force is put on it daily. See hand surgeon.

One more thought: Both prior answers are correct but: recommendation: get x-ray to rule out avulsion fracture. If no fracture, surgery is not the first option. It is the last option. Most such ligament injuries take a minimum of 6-12 weeks to repair and sometimes longer. If pain lasts >12 wks or if you are in a hurry for repair, seek prolotherapy to fix ligament before surgery. Good osteopathic dr. Can help.