I think your battery was over discharged. The capacity of battery will shorten due to over discharge.
Full discharge means to adjust your cellphone to the lowest power consumption state and then you use your cellphone until the battery runs out of its power(automatic shut off).
After full discharge, the inner of Li-ion battery still keep a little of electricity which is essential for the life of Li-ion battery.
Do not let the Li-ion battery lie idle after full discharge. Due to the idle Li-ion batteries will slightly discharge with time, it will lead to over-discharge.
To know more about Li-ion battery, you can see [http://www.witrigs.com/blog/something-you-need-to-know-about-li-ion-battery-maintaining-charging-skills-common-misunderstandings/|this blog]. I think it will help you.