If it continues, go back to your doctor. I think an inflamed pancreas or gallstone problem can cause pain that is at the front and referred to the back. It is less likely in someone young like you though. If you do not drink alcohol regularly or take strong medication, it is unlikely to be that. However, I am not a doctor and a doctor should be better able to tell whether it is muscular, indigestion, or other. Maybe it is time to reconsider vaping though as it is worrying you and it does have harmful effects for some people
Hello~Since your chest x-ray did not show any signs of lung cancer, I feel very sure that you have nothing to worry about. I am thinking it could be a misplaced vertebrae in your back, and if this is the case, you will have the pain you are experiencing. I would see a chiropractor, he/she will be able to help and after a few treatments, the pain should subside or even go away completely, you will probably be given some helpful stretching exercises as well.
Another cause for your pain could very well be costochondritis, it can cause back, rib and even chest pain, again, a chiropractor should be able to ascertain if this is the case. Also, a hiatal hernia can sometimes cause pain as well along with GERD (acid reflux)
Anxiety can also cause pain, the reason being that when we are anxious or stressed, we are more in tune with every little thing going on with your bodies, so, that can magnify pain or any other issues as well.
I hope that you will be getting some relief soon and some answers, living in fear is awful, especially when you have a new baby to look after.
If the pain is still coming back, then do see your doctor. They will know whether you need further checks or not. It might be acid reflux or something. You could also have pulled muscles. It is unlikely to be lung cancer as the X-ray was clear
If your chest x-ray didn't show anything, it's not lung cancer.
It could be anxiety, it could be just lifting the baby over and over again.
It's definitely not cancer.
Have you talked to your doctor about your anxiety? Post-partum can make the anxiety worse, too. Even if it's just overuse or something, you deserve to find some relief from the anxiety.