I think my baby is constipated. She's had a dirty nappy (not including wee) once today but it was…

I think my baby is constipated. She's had a dirty nappy (not including wee) once today but it was very small, then all this afternoon she's been acting as if she's been trying to poo, but really struggling, almost crying sometimes too, and every time I change her nappy, there is nothing there. I've tried moving her legs like she's riding a bike like they suggest, but it's doing nothing?