Suggest Medication For Constant Blood In Stools With Constipation

I have had blood in my stool for over 20 years. I am now 39. I also have constipation whenever I eat beans, milk and now even oranges. 12 years ago I eat bean almost every day because I was in a boarding school, now I cannot tolerate any amount. Recently I feel weak and lethargic. Whenever I take the over the counter fiber medicine I get relieve and the blood goes away. What may be my problem