It was an "around the world" milkshake, so it consisted of all of the flavors. On the website, it gave the sugar content for each individual one, because that was the main ingredient, and it came out to about 68 g of sugar. I usually do not take in this much sugar and never in my life had a milkshake from there. It was also my friend's first time also, and she felt extremely sluggish and tired, but her heart race did not increase. I believe it was the sugar content, and I will never ever order an In and Out milkshake ever again!
I would avoid high sugar foods. Have you ever been tested for diabetes?
Your heart starts pounding very fast because it is trying to get more blood to the organs of the digestive track so that they can work better. Your body wants to regulate your blood sugar levels, because too much of anything is bad. So it pumps blood quickly to your digestive system to try to move the sugar through your body quickly.
Chances are it was because of the sugar, however, it could have been from something else. A way to find out is to find out what is in the milkshake from in and out, and using ONLY SMALL amounts, see which ingredient/s are responsible. Then you can do research and find out why this/these ingredient/s do this to you.
There are sugar substitutes out there and most of them are a thousand times more unhealthy than sugar. When NOT taking in moderation (which is almost impossible because everything just about has it), high fructose corn syrup is bad for people. You for sure want to stay away from anything that has aspartame, it is an extremely unhealthy sugar substitute.
Please keep us posted and good luck.