I don't know if peanut butter is a constipating food from direct experience. Healthline says for most, no, but if you are intolerant of it, yes, it could cause constipation. As far as IBS goes, Monash says it is not a trigger in a 2 Tablespoon serving size.

I would suggest foregoing peanut butter for a week and see what happens. If you see improvement, also consider the food item you put the peanut butter on.