What Causes Pain In Lower Left Side And Pressure In Rectum And Anus?

Hi,I've been having pain in my lower left side for weeks now someday's Ihave no pain at all other day's it's back Ihave a very mild case of IBD for wich I take asacol Dr has checked my blood and stool samples ,all normal Ialso have pressure at rectum and soreness around my anus Dr also did a rectal examanation and proctoscopy all fine ,have you any idea what this could be ....