iPhone 5C heating up quickly,help please.

Hello fellows friends!

I'm holding on to an iPhone 5C. I really love it loads but what is bothering me is the heating issue.

I use Facebook/Twitter or Whatsapp daily without fail, but just using it for less than 10 minutes , I can already feel the heat. It's starting to warm up.

I am pretty concerns about this and I wonder if I got a defective unit? I hope somebody will enlighten me. Thanks in advance.