Make an appointment with your Apple store and let them have a look at your phone, It does sound as though you have a case for replacement, They may however be able to replace the Button/Button Mechanism, Let us know how it goes.
Before going to the Apple Store, do a full backup of your phone. This way, if you get a replacement phone you can set it up from your backup and not lose anything.
I saw on the apple site that they are going to replace the iphone lock button because alot of such problems have been reported but the ip5s much have a serial number from a selected range that they have selected