FIrst - welcome to Juvenation - it is a great place to find support and ask questions, whether you are new or 'old' to T1 diabetes, but esp. if you are new. I am guessing that you probably were not feeling great for quite a while and had maybe lost quite a bit of weight either recently or over time - this was a symptom of the diabetes eating away at your muscle mass because it could not use the sugar in your blood for energy because you didn't have enough insulin. Weight gain right after diagnosis is pretty common and in most cases it is a return to 'normal' or healthy weight. Because your body can finally use the carbs you are eating for energy, it can rebuild your muscle mass and fat stores. This probably doesn't help much with the problem of your dress fitting - but you are much more likely to be at or near a healthier weight than before you were diagnosed.
As for your dress- if it is mainly muscle that you have gained - are your measurements a lot different (waist, hips, bust)? Measure and see. your dress shop (or you already) will have the list of what they measured when they figured out your size to order. I would call and talk with your dress shop and the person who will be fitting you. Explain you were just diagnosed and that you have gained some weight and what you are worried about. They will know what amount they can let out the dress (like an inch at the waist, etc.) I am sure they have had to deal with bride's not being the same size as when they ordered the dress (like getting pregnant). So the cause might be different, but the end result of figuring out how to get the dress to fit is similar. If they really think it is a problem - they'd be able to tell you if they can reorder the dress in a larger size in time for the wedding (not the cheapest option probably.)
I'd also talk with your endocrinologist or your diabetes educator and or a nutritionist that specializes in diabetes, to see if there are some diet and exercise things that you can do for the next several months to maybe lose some of the weight. Do keep focused on your health and your blood sugars. Your wedding is one day, and you can find a solution for the dress problem - your health is with you for a lot longer and you don't want to risk that.
Good luck and have a great wedding!