Stomach virus ? what can this be?

oki so i am cashier at a store and a lot of people come to me with a big stomach cramps * woman men * . and not to long ago my sister had a big stomach pain and she vomit most of the time. now this think happened to me i feel really sleepy and my head hurts last night i woke up at night and could not fall asleep because my stomach was in pain and then i had to go to a washroom because i stared to feel funny .... what can this be ? thank you