My 2yr old daughter has bowel/stool problems...could some tell me whats going on?

Every since she was born,shes had been severely constipated..very hard,sometimes bloody stoll.. Straining a lot....up until a couple months ago,someone suggested that I take her off as much dairy products as possible,especially milk being that she drank so much whole milk. So I did just that,no more whole milk. A day that later,no more hard stool. I was very happy. But now she wont stop. Every pullup once a hour or two has this very yhick almost black pasty stool. Its also very foul. Sometimes just a small amount.somtimes alot. Almost all pullups has this black pasty stool in it for the past couple months. Could you help me on what may be going on. If it helos she just turned 2 in may. Weighs 27.6lbs.....she doent drink milk.....Thanks