What could be causing this possible symptom that I've been having?

The first issue sounds like a "Charley Horse" to me. It happens because you are stretching out your leg (after it's been bent, while sleeping, for many hours) and it cramps up. You can actually feel the BALL/KNOT form in your calf. It's very painful. Everyone gets them (at times). I have been told that if you increase your potium intake (eat bananas), it's will alleviate the frequency of them. I've gone through periods when I got them consistently, night after night. And, your second issue. I believe, we all get "sudden jerks" (once in a while) as we're just about to fall asleep. It feels like you're falling off a mountain. Not sure why that happens, but it's normal. I don't think you have anything to worry about. I'm sure that your legs, brain & nerves are working just fine!