Protein Powder help!?

Hi Im currently takeing Amp Whey protein from Gnc. I take 40g protein after my work out. This Stuff got me constipated! I was useing muscle milk and never had this problem. I compared ingredients and Muscle milk doesnt contain dairy. Soo what is a good lactose free protein? is soy the only way to go? I dnt wanna switch back to muscle milk it has too many bad reviews..