What would border control do if I say I went to Serbia to see my girlfriend?

First of all, there is no such thing as border control. There is the Dept. of Homeland Security. Under it exist Customs and Border Protection. Which divides into CBP Officers, the guys in blue that check your pports at the airport. Or Border Patrol Agents. Border Patrol agents are the ones with green uniforms who stop people from coming into the country from the land borders and coastal areas near land. If you go to Serbia, just tell the CBP officers the reason and the truth. If they find out you are lying, you will be in deep s**t. If you are a U.S. citizen you should have no problems. If you are a resident, sometimes there are certain restrictions. Just dont lie and you will be ok. Also, DHS doesnt care about your relationships. They just care that you are not a terrorist or criminal.