Tire Pressure Light

So I recently bought a alpine white 328i. I was driving the other day and my tire pressure light came on. It had come on, but went away after filling tires up with proper PSI and getting the tire checked out. Driving yesterday the light had come back on, and away from home I checked the pressure with the portable pump I have in my trunk and all tires read the proper PSI, but the light is still on.



Anyone have a idea of why this could be occuring?



p.s. I am new to the forums so if this topic has occurred before please point me in the right direction!