I accidentally ate peanut butter that expired. i thought it looked a bit odd but i didnt really pay attention. am i going to get sick ? i checked the date after?

Maybe not. The use by date is something required of producers of food products by the FDA. It is a point up to which they consider the product will retain its taste and nutrition value. Beyond that date it may or may not retain taste/nutrient benefits. It doesn't automatically turn into food poison. Indigestion and or other gut issues would be expected if the material was really bad.

Expired peanut butter(over 1-2 months after opening) is very unlikely to make you sick. Rather it gets rancid,dry,flaky, crusty and doesn't taste great.