Yellowing of Cymbidium leaves

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Hi.

I have a standard Cymbidium ( Pink Champagne 'Old Vienna'), and I haven noticed that the leaves, starting at the base of the pseudobulb have started turning yellow. Not dying leave yellow as if they were going to fall off soon. I have it it a south-facing window, and on the windowsill right it front of the glass. Could it be a bit too cold, too much light, or not enough fertilizer? Any information would be appreciated ,as this is my first Cymbidium ( I have had it since last May).

Thanks,
Cam