metronidazole gel 38 weeks pregnant...

My doctor prescribed metronidazole gel for me to inject at bedtime for 5 days for BV. I'm on day 2 and each time I've injected the gel into my vagina, my baby goes crazy and I get these TERRIBLENESS cramps. Is this normal? Should I stop treatment? Was my OB right for prescribing me the gel instead of the pill form? Will it cause me to go into labor? Please help me I'm really freaking out, these cramps are killing me. Lightning crotch.