Diflucan and bm

I am 26 weeks pregnant and doctor found that I had a yeast infection. She prescribed the one dose Diflucan, assuring me that it was safe in the later half of a pregnancy. The morning after taking the pill, I woke up with the worst stomach pains I have ever experienced in my life. It felt like someone had punched me in the gut. The only relief was when I'd have a BM. The pain lessened each day and as long as I didn't have any dairy products, I felt OK by the 3rd day after taking the pill. I read that lactose is in the pill - I am semi-lactose intolerant, so I'm assuming that is the cause for the incredible stomach pains. Also had a pretty bad headache the first two days.