BMI is 21 and WIC says overweight..?

I'm just really nervous right now after my first WIC meeting. At my doctor last week I was told my BMI is 21. I'm 5'4 and 127lbs, currently 21 weeks pregnant. It's already nerve-wrecking for me to be gaining weight, because I've always been really thin and athletic. But Friday at the WIC appointment, they told me I have put on too much weight and need to lose a few pounds. They scheduled me to see a dietitian in two months and also gave me brochures about losing weight. It was really embarrassing and I just want some kind info on this. I have gained about fifteen pounds during the pregnancy so far, is it really too much? Should I be dieting?

I have an appointment with my regular OB next month, but I'm going crazy over this until then. I'm nervous to be dieting, because I was already so thin when I first became pregnant and I don't want to cause the baby any harm.