I don't think this is too worrisome and would guess that when you are in this particular position, there is nerve compression that causes the numbness. Pins and needles and numbness are usually a sign of nerve compression. Your doctor said this is not of concern, correct? Check this out related to side sleeping from a chiropractor. https://www.premierchiropracticclinic.com/why-do-i-have-numbness-while-sleeping-on-my-side/ You aren't alone.
We get into habits with our sleep position. Side sleeping has a down side. But I know it is hard to change that. However, would think this is impacting you in the issues you are having. They aren't serious but more just like a nuisance.
If your doctor is a GP, of course, he wouldn't know enough to properly diagnose you either way. You don't say what your age is, but the reality is, we don't know what we don't know, and doctors don't know what they can't diagnose. I'm not seeing cancer here, but you're right to be alert to the possibility because when it comes to cancer the earlier we catch it the better chance of leaving it behind. I'm thinking that if it were cancer the numbness would be there at other times as well, as a tumor would always be there, not just when you're lying on your side. Most likely it's inflammation in the hip, perhaps bursitis, or something like that impinging on nerves. But the only way to be sure is to see the requisite specialists and get all the requisite testing if you're concerned enough to go through all of that. Do you have hip pain any other time, such as when walking long distances or running? That would tell you something. Do you have lower back problems? That can impinge on nerves as well. But you'd notice because you'd have pain in the lower back or the hips. I have a lot of trouble up and down because of problems in the back, hips, and neck, but those things hurt and so I know where the pain is coming from. I've had diagnostics done. If you've only seen your regular doc and not an orthopedic surgeon or an oncologist, you'd really have no way of knowing except his experience saying it's not serious rather than actual testing. My Mom had her first symptoms of cancer, or so I've been told, with periodic numbness in the legs, but it was while she was doing stuff, not just lying in bed, and her cancer was in the spine. Which is why I say, you're right to consider it, but probably wrong about it being the problem from your description as that's not a super common place to get cancer and if you're young, unlikely. But you're smart to consider what it might be until you find out what it is. And if you have hip pain, that's the most likely problem. Peace.