Fifth Harmony had only 4 members who were free last night, because Lauren Jauregui was busted for weed Tuesday night while trying to get on a plane at Dulles Airport.
Law enforcement sources tell TMZ ... Lauren was stopped at a TSA checkpoint and forced to submit to a secondary search, when a TSA staffer found what appeared to be a bag of marijuana in her carry-on luggage.
Cops were flagged down in the terminal and they promptly arrested the singer.
It appears Lauren was heading for Brazil where the group is set to perform Wednesday night.
9:20 AM PT -- Lauren's attorney says the singer was not arrested or detained, but was "simply given a citation for possession of marijuana and released on her own recognizance." She adds, "This will not interfere with any future scheduled performances or activities."