Could be a number of things happening here. Hard to tell without more information.
Does the brake pedal go complete down to the floor?
Have you checked to make sure there is enough brake friction material in the brakes?
Have you checked the level of brake fluid in the brake master cylinder?
Are there any unusual noises accompanying braking, scrapping, chirping, screeching, pedal pulsations, etc.?
Does this happen after the car has sat overnight?
Does this happen after the car has completely warmed up or if you have been using the brakes heavily?
Could anything for worn brake pads, heavily contaminated or low brake fluid levels, faulty calipers/wheel cylinders, worn rotors/drums, faulty master cylinder, damaged brake lines, etc.
If the brake pedal sinks all the way to the floor, you have a serious brake issue and need to get this looked at ASAP. Car may be braking OK now, but that could suddenly change. Most common cause of a sinking brake pedal is a faulty master cylinder. Usually caused by brake fluid that has been neglected for a long period of time. Over time, brake fluid will absorb moisture, this contaminated fluid will cause corrosion to metal components in the brake system, damaging seals and pistons.
Sometimes, it is due to the rear drums (assuming rear drums, front disc brakes) being grossly misadjusted. Usually after an inspection or if the brakes were recently worked on.
Since the car is "new" - I wouldnt guess at the issue, take it in, as it is still covered under the 3yr/30K mile bumper to bumper warranty. Start a workorder on it, start a paper trail to protect yourself.