Battery on Kobo Aura H2O went from 30 % to 0 % very fast

Hi

I have just got a Kobo Aura H20 which I am generally very happy with. The battery life has been extremely good and one charge has lasted me several months. However, I just noticed that when it reached about 30 %, in about a day it went to 0 %. Is this just the battery indicator that is buggy or do I have a problem? It's slightly annoying that I cannot count on the indicator. I mean the first 30% took maybe 3-4 weeks, the last 30 % took hours...