How do you know it is the plant based butter and not the taco bell or something else you ate?

You can't generalise "plant based stuff" like that. There's so many ingredients and foods that are plant based and they all have different effects on your body. Even if it is the butter, that doesn't mean all plant based food will have that effect.

Going fully plant based will usually increase your fibre intake. But then you also need to drink plenty of water to avoid constipation.