It did happen. My wife had two times of this experience. The most important thing is to rest and keep yourselves relax ..... If you see the "brown" thing, it's better to go to see the doctor and she/he will help you to check the heart beat of the fetus. If the heart beat is still there, it's okay. The doctor may give you some medicine to stablize it.
Rest .. Rest ... Rest is the most important thing... Also, don't pressurize yourselves...