Weird dr appointment

Went my dr today to "discuss" my ultrasound, which I thought was weird. Well she said baby is healthy checked the heart beat..136, all is good. I told her I've been very winded lately. Mind you , I'm pregnant(obviously lol) I have an 11 month old and I'm working evening shifts. But she told me she wants to do my diabetes test at 24 weeks ... Why would she want me to do it so early ? With my first I didn't have pregnancy diabetes