Back Ache, Blood In Stool. Occasionally Have Constipation. What It Could Be?

Hi, on and off for about a year I get bouts of blood in my stool , when it is bloody the water is usually bright red with an average size stool with blood mixed in, only after I ve passed the stool is when lower back ache kicks in and can last for hours, this can happen for up to a week and then I might not have blood in my stool for months, but the lower back pain can be more common, usually, but not always, after a bowel movement . It can sometimes be accompanied by constipation . Any ideas what it could be? I m a 20 year old male