2 glucose tests?

I'm 21.5 weeks and My docs office just called me and said they wanted me to do a 1 hour glucose test before my next ultrasound with my high risk specialist on Thursday. I'm not technically high risk just been seeing one since I was bleeding in the second trimester but everything seems fine now. The nurse then said I'll do another glucose test around the normal time at 28 weeks. This is my third pregnancy, first two I passed the 1 hour glucose tests just fine. I know anyone can get GD regardless of perfect prior pregnancies or not but I'm confused why they want one done now and then another in almost 2 months?? Baby only measured a few days ahead at my 16w U/S. has this happened to anyone else? It's got me worrying now😩😩