5 month old sudden constipation??

Anyone knows what could suddenly make a 5 month old start suffering from constipation? No diet changes have been made. Its been going on for like 2 weeks now. First time he didnt poop for 3 days and i gave him a little juice like my doctor told me once when i use to think he suffered from constipation only to find out he just normally pooped every 2 days which doctor said was fine as long it wasn't firm.

And he had went about 3 days no poop at 3 months and i was nervous. Doctor checked his stomach and was like he just didnt need to poop.

Anyways 5 months old now, 6 months on the 2nd. so first time didnt poop for 3 days. Gave him juice at night. Pooped firm first the next day but then soft all day multiple poops. Then didnt poop for 2 days and pooped soft. Now didnt poop for 3 days and it was firm again and he seemed in pain, once he finished pooping though he was fine. Was firm then towards the end went soft.

He's always drank the same formula and hasn't started solids yet, belly isnt hard or swollen, poop looked normal other than being firm. Never went through this with my first so its new to me