when stepping on brakes, they go all the way to the floor

I have only had this car for 2 weeks and 1st noticed brake light coming on periodically. Then noticed brakes would first be hard when pressed on and then they would sink in as though they were not going to stop. Scary when you are going 80mph with cruise control on (had a road trip and on highway) and all of a sudden had to slow down immediately. Has anyone ever experienced this before? Took it to dealership, they said the computer would tell them all lights that came on and see if brake lights detected any problems? After having it all day, only to tell me they had not detected any problems nor did the computer alert them of any brake indicator lights? Very concerning...