Knee/shin pain?

Hello,

I have been experiencing some knee pain for a while now. It feels as though there is some kind of "air bubble" that needs to be "popped" in my knee cap. I do not like bending my knees anymore since it kind of hurts and the thought of doing a squat makes me cringe. My shins also bother me sometimes, and I feel like I've been getting shin splints solely from walking around for the past year and a half. I used to be very regular about exercise (and maybe overdid it sometimes), but haven't honestly been working out regularly for the past year and a half. I take a multivitamin daily so I feel like I definitely get enough calcium, so I don't think this is necessarily related to that. I am almost wondering if this is from lack of activity? I never used to feel this when I was working out for 1-2 hours 6 days a week, and now my exercise is really just daily life, and maybe going to the gym once a week if I'm lucky. I really want to start running outdoors and be able to actually run a mile (and more eventually) without stopping, but I feel like this is getting in the way and I have noticed some pain in my knees and shins. In fact, I am sitting down right now to type this not moving at all and I do feel a bit of pain in my knees. Anyone have any insight? Could this be leftover from a history of overexercise or weakness from prolonged lack of activity?