Heavy use (and therefore repeated charges) will lower the capacity quite quickly. Normal smartphone battery may lose capacity significantly after 500-600 cycles. You may read this article on battery life and battery capacity vs discharge cycles (http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries)
It is a good practice to full discharge battery (and then fully charge it) at least once or twice each month so that the indicator of battery levels is more accurate.
Anyway, you may try to get a new battery if you use it heavily.