Battery and Lightning bolt underneath

Last night I did not get a chance to charge my iPhone 3GS before I went to sleep. I wake up in the morning and notice that my battery is dead. I plug my phone into my computer to charge and I get picture of a battery with red filling a very little bit of it up with a lightning bolt underneath, I know that this means the phone is charging, but it has been on this screen for over 6 hours already. What's going on??