Macbook charger red light but still charging

I have an older MacBook Pro (early 2011) so I know it's probably on its last legs. When the charger is plugged in, the little light goes from green to red pretty quickly but its still charging the laptop. Is that a signal that the battery is about to die? Is that a warning of some sorts? While the battery condition says 'normal', it does have over 1200 charges and apparently it's only supposed to last about 1000.