A solid red light would mean charging.
Blinking red means that there's a problem with charging. It could be overheating (if you run games while charging, for example), could be a problem with the charger, could conceivably be a problem with the phone. We'd need more information to really diagnose and suggest a fix.
In case it's overheating, try just letting it cool before attempting to charge. Otherwise check there's no fraying of the charger cord, or maybe try with another charger (if you have one) or via usb (or if it's happening when using usb, try with the wall charger or even a different usb port).
Otherwise you could post again with more detail about the phone and when you get this problem, or if you prefer register and post in the main forum where we can discuss it better, and hopefully we can identify the likely cause.