Try using it for a while and see if the battery lasts as long as it used to. If it still works ok then there's nothing to worry about.
On the other hand, it is possible that the battery is starting to lose its capacity. When they go, they tend to have shorter discharge / recharge cycles, so it will seem to charge up faster but will also run down faster too.
It's also possible that your computer's usb port isn't putting out enough current. Some usb ports only provide 100mA and that might not be enough for the DSL's charging circuit. If you can, try charging it from its original adaptor plug and see if that gives you a better charge.
Cheers!