Which vendor was it from? I've had it happen in the past where a vendor will forget to agitate their stock bottles and I will get an aliquot of really non-viscous flavoring and PG, absent of most nicotine. It's possible most of the nicotine and VG settled to the bottom and yours was not properly mixed.
It could also just be a function of temperature. It is summer now and you are storing your solution in an equally 'hotter' environment
