I would suggest that you have it looked at by a Dr. But generally if you can not put weight on it without serious pain, than it is serious, the degree of seriousness needs to be determined by a physician. Please click on one of the stars next to RATE THIS ANSWER, and I would be very grateful, hope your knee is not serious dear, ouch...*p
A grabbing feeling is more of a muscle issue then bone, it can possibly be a tendon or arthritis. My personal chose would be a physician.
If you have received even a minor injury during a Car accident, then I think that you are entitled to receive some kind of compensation. It is better to always have the contacts of a reliable and proven lawyer, because it can really come in handy at a time when you don't expect it, but it will save you unnecessary anxiety and panic.