Was it the 'alternator failure' light (normally placarded 'low volt' on the instrument panel) or a light with 'low volt' on the lens or similar?
The former is fitted by Cessna as standard equipment and is actually an alternator failure light which may be due to an over volt situation (causing the alternator control unit to go off line) or an actual alternator failure. These are normally on the right hand side of the panel out of the direct line of sight of the pilot...
The latter is a UK requirement and actually measures busbar voltage rather than alternator status. Once the alternator output fails, it doesn't take long for any service (beacon, radios etc) to drop the battery voltage below that at which the low volt light will illuminate.
If it was a transient fault then recycling the 'alt' part of the battery master switch may bring the alternator back on line.
You did the right thing by landing. The pulsing of the ammeter (showing a discharge in this case) may be due to the anti-collision beacon - they don't show anything much at the best of times. The minimum in the UK for battery life is 30 minutes worst case scenario - IMC and night. Best to reduce the electrical load as much as possible and land as soon as practicable.
The fault could be the alternator control unit (remote from the alternator and a solid state voltage regulator/over volt relay), the alternator itself or something in the wiring. There is a fair bit about the C152 electrical system in the POH and their should be a supplement to cover the separate low volt light.