zofran

My dr gave me zofran for my severe morning sickness and I've been taking it a little over a week now and it was working miracles I hadn't been sick at all. I was feeling almost 100% better and had my appetite back but as of yesterday I've been getting the morning sickness back I threw up once yesterday and several times today. Has anyone else had this issue while taking it? I'm just confused because it was working so well i dont know what changed :/