Q: iPhone 5 gets pretty warm when it is charging

Is it normal that whenever i charge my newly replaced iPhone 5 battery that it gets pretty warm when it is continous charging? i just recently got my iPhone 5 battery replaced after using it for almost a year since it was first unveiled last year, because i have been having troubles with it since i upgraded to iOS7 and one of the genius bar people told me that my battery has been swollen and it was on the red zone and i paid for a new battery toget it replaced, now that i am charging my phone with the new battery in it and it feels like that the surface of my phone gets pretty warm.  any suggestions and comments are greatly appreciated guy!  Thanks very much