Tomorrow is my due date

Why are my cramps worse when I sit down?! I feel pressure on my rectum, and crampy achy back and tummy it's more annoying than painful. Like makes you wanna squirm around! I've had it for 3 days now. Labor soon? 40 weeks tomorrow! Also feels like it lasts a long time so I can't really time anything? It's more of a constant feeling :(