Hi,
I assume you are using VCC as reference voltage.
Because of the known problems with drifting VCC this is not recommended for absolute voltage measurement with reasonable precision.
There is a good reason why they spent a pin as reference voltage input.
--> Connect a reliable, precise and stable voltage reference IC to it.
Measurement means "compare with a reference".
If you want to measure your reference voltage this means you need to "compare" it with a another - more reliable - reference.
Do you have such a reference? If yes, then why do you use VCC as ADC_reference?
--> If your reference is not reliable then it is no reference.
Some microcontrollers have a built in voltage reference. If your microcontroler has one, then use this reference instead of VCC - always.
Klaus