I agree with everything Red has said. Your doctor should have set your peak flow standards when you were diagnosed. You definitely need to see your doctor again. A peak flow of 150 is low. If it's been a while since you've had an attack, you probably need to be re-evaluated any way.
Not long after I was diagnosed, my allergist taped my peak flow zones on the back of my peak flow meter. 400 or above is my green light zone, 380 to 220 is my yellow light zone and 200 or below is my red light zone, which means I'm headed for trouble. In case you end up in the ER, it is also good to take this with you so they will know what your normal peak flows are.