Baby pooing problems.

Krista
So my 18 month old has had problems since she was born, but just recently it has gotten out of control. For about two to three weeks she has nothing but diarrhea no matter what she eats, then she has is so constipated she screams when she poops. I'm talking golf ball size poops when she's constipated like she is now. I took her to the doctors and the gave her a laxative which she ended up being allergic to so they told me to up the amount of fiber and not as much milk. Well I have tried everything and she's still constipated. Only two bottles of milk a day and the rest of the time water, no breads or pastas, and even and over the counter suppositories. Nothing I'd working and I just feel so bad for her. So I was wondering if any of you had experience with this and knew something I could do for her.