Most likely, you are simply experiencing muscle fatigue in your lips. Don't sweat it too much right now. Whenever you notice the trembling begin, take a 10-15 minute break and then go back to playing again. You find that the trembling has subsided and you can play for a while longer. If it still continues, stop for the day. Over time, you will build up the muscle tone again and be able to play for longer periods. It's still a good idea to take regular breaks during practice sessions. When any muscles become overworked, they may exhibit various signs of fatigue. During strenuous gym workouts, it's not unusual for arm or leg muscles to tremble when overworked. Lip muscles are no different than any other.
"Never give a flute player a screwdriver."