Stiff baby

I noticed very early on that my baby would become stiff like an ironing board. I realized it was when he was tired and it happened less when I focused on getting him to nap earlier. However now that he's 4 months and rolling and generally being more active,I feel like it's gotten worse. He will stiffen his back and arms and legs when he doesn't want to do something or when he is tired. I googled it and now I'm freaking out that he has cerebral palsy. He's never been a cuddly baby...