I am lactose intolerant and I have no problem eating cheese, ice cream and yogurt in moderate amounts. Drinking milk, even goat milk, or putting it on cereal gives me gas, cramps and diarrhea. I don't know why. I have read that many people who are lactose intolerant can tolerate cheese. I know yogurt has enzymes that help digest it.
I have tested allergic to chocolate. I can have it infrequently without an upset stomach. Had you been eating chocolate often without ill effects? If so, this chocolate could have put you over the edge if you're intolerant or allergic to it.
Same with the milk. If you'd been consuming dairy products the chocolate milk may have been one drink too much.
I can drink lactose free milk without difficulty. I have not tried drinking it every day. I use rice milk.
I am allergic to several foods. Lactose causes gas and diarrhea. For me, food allergies cause an upset stomach and diarrhea but not necessarily gas.
Your symptoms do sound like you're lactose intolerant.