Painful poops

Baby boy will be 4 weeks on Tuesday (yay!) and I have noticed that he becomes really rigid after eating lately. He arches his back, cries/whines, kicks his feet, pulls them up, and sometimes pushes them out as far as they go. He makes grunting and straining sounds that sound painful, and his face gets all red. He’s completely restless and inconsolable, and has bouts of it until he finally releases the poop; then you literally feel his body relax and go limp.

The poop is not solid or hard, but pretty loose and runny. There is no blood or mucous, but I wouldn’t really consider it mushy because it doesn’t really have a form. The color is a yellowish-brown.

Back story to explain my worry—my oldest has GERD and was put on soy. Later on we learned he had a milk protein allergy and that also contributed to his feeding issues with regular formula.

I’ve read that at this age, some babies go through his normally because their digestive system is working itself out. We have switched his formula from Similac Pro-Advance to the Pro-Sensitive to see if that helps, but haven’t seen a significant change to warrant significant improvement. We have used the new formula for about 2 days now. We have one more sample can before we run out. We have one sample can of the Similac Total Comfort to try as well.

My question... I don’t know what I should do. I don’t like to see my baby in such pain, especially when it’s caused by something I’m giving him. Should I finish out the remaining can and see if it changes? Or should we try the comfort? Or should I try soy? Or is this completely normal and ride it out? I’m afraid the doctor will tell me this is normal and it not be. Help?