Unwanted comments

I'm currently 18 weeks pregnant and am having concerns about my size. One specific coworker comments on how small I am/that I still don't look pregnant EVERY TIME she sees me. She went as far as to tell me "I guess if your OB says the baby is growing ok, then you're fine." I don't think she's intending to be mean, I just don't understand why she thinks it's appropriate. Does my bump look normal for the stage I'm in?