Irregular Bowel Movement

Hi, i have a daughter of 2 yrs of age who has been suffering from irregular bowel movement for the last year. On many occasions I have been to see the doctor who have sent me away with a prescription for lactose medicine this has not helped the problem so my little girl continues to suffer. She can go days without opening her bowels, when she eventually doe's go it bleeds. The build up to her going is always painful to watch, she is in constant pain. We have tried several things to try and make it easier for her to go even changing her diet. Were at are wits end and need some advice. PLEASE HELP :-(