Hi thanks for you question.
Possible causes for bleeding per rectum are;
No 1.
Chronic constipation can cause injury to rectum,which can bleed profusely.
The second possibility could be
anal fissure.
Third possibilty could be heamorrides.
Fourth could be
rectal polyp
Fifth and final could be cancer of rectum which can also give rise to heavy bleeding per rectum.
I would suggest you to
consult a general surgeon who may examine you and may perform proctoscopy/sigmoidoscop on you to determine cause of bleeding.Light headedness could be due loss of blood.
Hope this answers your question.