Alternator problem

The other day, I was flying and noticed that my ammeter guage was reading 0
(it had been reading fine for a long portion of the flight.) Also, I had
been running a lap-top from the cigar lighter plug for the first time ever.
I turned-off virtually all electric devices and continued to my destination.
I also shut-off the alternator switch. Later in the flight, I tried
recycling the alternator switch, and the guage would read full deflection,
so I then would shut the alternator switch off again.

Later in the day, I fired-up the plane - could tell that the battery was
weak because it could hardly pull the prop through. The ammeter guage,
however seemed to look ok - appeared to be charging the battery. I figured
that somehow it had recycled itself, so I took off and headed home without
incident (I watched the ammeter guage very closely throughout the flight,
and it appeared normal throughout.)

The plane is a 1974 Piper Warrior - 151.

Any thoughts? Was it merely a failsafe due to too much current being drawn
from the lap-top? Should I have it looked at?

Thanks