Poop help!

My 6 month old has been ebf, but recently started trying baby food and blw. I noticed he started grunting and straining, so we went back to ebf...except for a few puff here and there. The last few days he's been super fussy and still grunting and straining. We stopped giving him puffs, but nothing seems to help. Yesterday, his poop was brown and super stinky. Today, he screamed after he pooped and screamed while I changed him. He doesn't have diaper rash or a fever or anything else. Any ideas?