Do you think you will be the only girl on her period at the party? You won't...and it's a part of life, and you just have to accept it. Now, what to do while at the party. If you are wearing pads, then continue to wear pads, because you will be more uncomfortable with just trying it for the first time. That being said, you just have to be precautious...and the wearing of tight shorts under your dress is a great idea. It will give you a little more comfort. And...aspirin
thins your blood and your flow will be heavier, so if you can withstand taking aspirin, then do so. Best of luck to you, and be yourself, and have fun. Being on your period is not a curse, it's a part of life. You will soon realize that.