Really bad lower abdo pain after opening bowels & pain in rectum so bad i can't walk & have to lie down , why is this ?

See below: I think you mean anal pain when you say "pain in the rectum".Anal fissure is a common cause of this symptom.It is a tear in the lining of the anal opening.Treatment is usually non surgical involving high fiber diet & stool softener.Have a doctor check you because it is impossible to diagnose your condition online.It could be other things.I hope you feel better soon.

Hemorrhoids: Rectal pain can be very severe and your descriptions sounds the part. Many causes can contribute to rectal pain, commonly hemorrhoids , fissures and maybe infection. Considering your pain is so bad you can't walk or lie down urgent visit at the Emergency Room is recommended. Good luck.