Sudden gas pains, very bad lower abdomen and pelvic pain, first poop was constipated like, then 2nd poop was extremely loose. all i ate today (in the last 4-5 hours) i had a sandwich. what could this be?

Cramps: It's too soon to tell whether you're developing a viral intestinal tract infection or reacting to something you ate hours earlier. Eat a soft, bland diet with no roughage. Food poisoning or intestinal viral infection symptoms will gradually improve on their own. You can take kaopectate or Imodium if loose movements become excessive. Avoid dehydration. See your MD as needed.