when a lion chases a gazelle for a half mile and then gets thirsty what does it drink?
no animal on earth drinks anything but water.
all these beverages from milk to soda, to gatorade, to tea, to coffee, to juice to whatever is not what we have drank for 2M years or what a single animal drinks on earth. it is just marketing for companies to make $$$. but even with water they came up with the idea for bottled water cause they couldn't even let us have that
but to answer your question simply, diet soda contains a lot of chemicals and bullshit in them. i don't care if it has 0 calories! doesn't mean shit to me. when your body is under stress and you throw something at it this is what will happen everytime, it will want to get rid of it right away cause it doesn't want the extra stress. SIP your water only, and a lot of runners including me sip and spit. your body still does some work when you gulp water too. and i've seen plenty of runners including me throw up that water by drinking too much before or during.
so after a run like that sip some water, and take a cold shower. that is my advice