Aw I had the same a few days ago, I had this sore pain in my tummy on the side and it would go away when I laid on my back but would come back when I slept on my side, it definitely felt like my muscles were stretching. It's amazing what our body's go threw!
Every week can be different hun , as well as every pregnancy . I'd say its your ligaments stretching and your body just preparing itself , unless it becomes unbearable and there's any bleeding I wouldn't worry . Congrats .