Hello,
the weight gain isn't a problem as long as you are eating healthy and your uterus (and baby) are growing appropriately.
When I was last pregnant (I am diabetic), they didn't want me to gain more than 16 lbs the whole pregnancy.
Let us know how you go with the additional testing. In the meantime, avoid sweets and too much carbs and focus on eating plenty of veges, meats, etc.
Even if you were underweight at 33 week 16 lbs isnt horrible. I've gained the 8 pounds I lost in my first trimester plus 1.6 lbs more and I'm 30w5d
Its not that big of a deal they will make you take a 3 hour glucose test and if u fail that one they will most likely put you on a special diet. Try not to worry to much it usually doesn't mean anything