It's almost certainly the charger - or the USB cable, if the charger uses one. Basically, micro-USBs are very delicate and easily damaged. I think the cable on my HTC charger lasted a grand total of 4 charges before giving out.
Fortunately, chargers are cheap and USB cables even cheaper - at least on ebay, which is where I get mine.
When you get a replacement charger, be sure to check that it provides the same (or higher) number of Amps as the original. If you get one that's less, you could find that, if in use, the tablet will lose charge faster than it gains it. It'll also charge a lot slower.