My iPhone 4S becomes warm after downloading GPS app

I need immediate help on my iPhone 4S. I downloaded a GPS application and started using it without problems. But while I’m trying the application, I got a phone call and totally forgotten that I have an open application running on the background. In a few minutes, I notice that my iPhone becomes moderately hot. Was it the GPS application or is there any other reason behind it? Please let me know. Thank you.
