If you do not have an effusion (swelling actually inside the joint), the chances of you having torn the ACL or PCL is pretty low. Both of these ligaments have an artery running along their surface, so if they are torn or ruptured, that little artery gets torn also, and bleeds into the joint.
However, you could have a Grade I or maybe even a Grade II ACL or PCL injury (tweeked it some).
You could have also stretched the posterior capsule of the knee joint in the back of the knee.
Another injury, not really known about until the advent of the MRI, is a bone bruise. When the tibia and femur get slammed together with enough force, it can actually bruise the subchondral bone, causing there to be edema within the bone. Thankfully, these heal on their own.
It's also possible to have pinched and torn one of the meniscal cartilages. Since these do not have a very good blood supply, they do not bleed when torn. Only the meniscal injuries, where the meniscus gets torn off the capsule, bleed. So, that is another possibility.
Since it hurts to straighten it, you may have injured the posterior capsule. This will heal on its own, but it takes a little while. You do need to make sure you get your knee completely straight at least a couple times a day. About the only problem that arises from a torn posterior capsule is if it scars and shortens. If that happened, you would not be able to straighten the knee completely.
If the injury is slowly getting better, then you are probably okay. Just get your range of motion and strength back. However, if you continue to have problems, then you may want to see on orthopedic surgeon.