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.