Every pregnancy is different. Every baby in the womb moves differently, at different times, different intensities. Every woman has a different pregnancy, no matter how many children she has had. I know, I'm pregnant with my 4th. The back pain you are describing is normal as well. Ask your doctor, I'm sure he/she will say so. I had severe back pain with my third child. It was on my right side, about where my ribs are and spanned across my back and down my lower back. At times it was so bad, I had to lie down on my left side just to feel relief. Fast forward and I'm pregnant with the fourth and about the time of Thanksgiving is when my back pain began and it was the same kind of pain I experienced with my third. I've seen a Chiropractor twice so far since I've gotten pregnant, once when I first found out and again about a week after the pain began. I have not had any pain since. I'm going back again in a week or so. I would suggest you see a Chiropractor. My Chiropractor told me that back adjustments will make a world difference with the back pain. And he was right!
The internal stretching you feel in your lower abdomen also sounds like what I have been feeling. Your uterine ligaments, muscles are stretching. Your doctor should also confirm that this is normal. I've experienced it with 3 pregnancies.
So, do not be worried about something that has no reason to be worried about.