baby strains and arches his back in sleep

My son will be 6 weeks old on Monday. About 2 weeks of age he developed sleeping problems. He strains and grunts and arches his back in his sleep. He almost never wakes up doing it, but his face turns red and he is very uncomfortable. It starts around midnight every night and gets worse as the night goes on. He stops doing it around 7-8 in the morning. Sometimes he wakes up crying but it`s very rare. Lately he started doing it also when he is awake and naptime during the day. Of course I haven`t slept either, because I can hear it and really breaks my heart seeing my baby in pain. It seems like he is constipated, but he has normal bowel movements. My pediatrician says it`s normal baby behavior and he is colicy. But he is not fussy and doesn`t cry while doing it???? He is a happy baby while he is awake and really only cries when there is something wrong. I also thought it`s gas, I have cut out all dairy in my diet and switched the formule to soy formula - I breastfeed and formulafeed, since I don`t produce enough milk. Nothing has changed. So I don`t know if it`s gas, if he is colicky or is it something else. Has anyone else had these problems with their baby? Please help, I don`t know what to do.