Knees Ache and Stiff when Sitting for Short Periods; Arthritis?

[jammy21]

I've been getting moderately stiff and achy knees when sitting the last few weeks especially when my knees are in the 90 degree position. Both knees are effected evenly; and this feeling sets in as little as ten minutes after sitting. Its almost the same type of feeling you get when you havn't moved for many many HOURS, and this feeling of needing to stretch to alleviate the rigarmotis. When i get up, walk around and stretch the feeling goes away; but once i sit it returns again.

Does this sound like some kind of arthritis setting in? Im in my 30's; there doesnt seem to be any swelling, i have full range of motion.