Zofran and vomitting

I took Zofran after a terrible Migraine with severe vomiting. I was given it in the hospital and then I was on it for about 4 days. I was constipated the whole time but didn't realize it was a side effect. Now over two weeks later I'm still constipated and I've been off the Zofran for the 1 1/2 weeks. I have to take this Senna-gen lax every evening in order to go to the bathroom. If I don't take the Senna then I don't go. I've tried going off it for 2 days at a time. I get a great vegetarian diet...which is full of fiber. I'm very unhappy that I still have constipation. In fact, my doctor said that I have Irritable Bowel Syndrome now. Please help if you have any feedback. L