Along with growing pains it's entirely likely that it could be gas too. Gas hurts in a completely different way when you are pregnant compared to when you are not pregnant. Either way it is completely normal to have pains now and then. If the pains are accompanied by blood (enough to fill a pad in an hour, not just spotting) or if the pains get stronger and more frequent you should go get checked out again. But for the most part it sounds like round ligament pain or gas.
This happened to me two days ago. I agree with your doctor. It was definetly a good pain in my lower back. I had to just lay down and support my belly. I'm 15+4 days and it helped. I only had it for one day.
I'm pretty small and that happens to me often, too. I have to remember to move slowly, like when bending down, standing up from a chair/the car, turning around, etc. It feels much better if I remember to just take it slow. Hope you feel better!