What could be the cause of severely painful rectal spasms?

Proctalgia fugax: Sounds like proctalgia fugax, which is a benign condition. But it could be very painful. It's thought to be due to spasms of pelvic muscles, surrounding your rectum. It's usually very short- lasting and self- resolving. Heating pad or massaging the area could be helpful. But then again it could be due to other conditions. If it is bothering you a lot, it would be a good idea to see a doctor.

Stress: These spasms are often related to stress. Sometimes they wake people from sleep, but are gone so quickly that any treatment (like sitz bath, or pain medication) doesn't really help. Physical exam may identify spastic pelvic floor muscles, but is otherwise usually normal. Avoid fixating on bowel habits, stay on high fiber /fluid diet, try to manage stress better. See your doctor if it persists.