Could you explain me why acute pulpitis causes pain at night?

Vascular engorgement. Inflammation causes increased blood flow in the vessels of the pulp. When you lie down, this engorge ment can increase. The pulp is limited in space and confined within the hard tooth structure, so the increased volume of blood causes pressure and pain.
Yes. Acute pulpitis is an inflammation of the nerve or the pulp inside your tooth if it's just mildly irritated throughout the day when you're active and you're concentrating on everyday activities you may not notice the pain so much but at night when you relax and you're not thinking about everyday decisions minor aches and pains are more noticeable to the body.