Hi Babinogi, I had GD with my first pregnancy, and I too am tall and thin. I was told this doesn't necessarily have anything to do with whether you develop the condition; maternal age is definitely a factor. The most important thing: don't panic! Even if you have it, it does not automatically mean you will have a bigger baby or that you need to have a C-section. I had a natural vaginal delivery, no problems and my daughter weighed 7 lb 13 oz at birth-- hardly macrosomic. FWIW, my sister had GD too with her second child, and she was born at 7 lbs even, after a vaginal birth.
I second the suggestion of buying a glucometer and testing just to see what's going on. It's a far more reliable way of understanding how you're responding to processing sugar than the OGTT test. Test in the morning upon waking, and 2 hours after completing your meals.
Finally, just wanted to add that if it turns out you DO have GD (and you may not), if you are able to control it with diet and exercise, you are considered no higher risk than any other pregnant woman having a "normal" pregnancy. This is super important, because a well-meaning OB might try to pressure you into induction, NSTs, C-section scheduling, etc, "just to be safe", when in fact the protocols do not call for it. I had this happen to me in my first pregnancy and switched to a midwife at 35 weeks (it's never too late as it turns out!) who felt as passionately about this issue as I did. That turned out to be the best decision I could have made. If I had stayed with my OB, I think I would've ended up with a C-sec for sure, and that would have been less than ideal as I can't have an epidural due to a spinal rod, and would need general anesthesia for a C-sec.