Q: my iphone 5 is always a hot. What is problem?

Hello as_ramunas,

Depending on the warmth of the iPhone, this can be normal behavior. The following article explains warmth while charging and using other functions more in-depth:

iPhone, iPad, and iPod touch (4th generation): Keeping device within acceptable operating temperatures

http://support.apple.com/kb/HT2101

Your iPhone will warn you if it is over heating, and if it needs to, it will shut itself down. As long as you have not experienced this warning, what you are sensing may be normal. If you believe the temperature is a concern, you can isolate the issue by restoring your iPhone as new.  This will help you identify whether it is a hardware or software issue:

iTunes: Restoring iOS software

http://support.apple.com/kb/HT1414

Thank you for posting in the Apple Support Communities.

Best,

Sheila M.