U ever heard that it's not good to mix beer and hard liquor??
For a person u age and ur sister's age, I dont think there is someone seriously wrong, but if that happens every time u drink, I think the best bet is to not mix alcohol and see if that's what making ur stomach hurt.
If so, problem solved. If not, go see a doctor...
And females who actually want to be able to give birth to healthy babies and have no liver problems at a later age, it's wise not to drink!