Taking Melatonin while 32 weeks pregnant?

I went to my doctor yesterday and she told me to start taking Melatonin because I have not been able to sleep. I took it last night and I ped right out at 9:30, but I woke up around 1 to go to the bathroom. I could barely walk it was like I was drunk I was all over the place. I woke up again around 5 and it was the same thing. I woke up about 8 this morning and have been in a haze all day today. I'm afraid to take it again because if it does that to me what does it to do the baby. It says do not take if pregnant but my Ob told me to. Has anyone else taken it? What should I do???