My lock button is broken.

My iPhone 4 lock/sleep button has been fine for the past year and was working fine earlier this morning but this afternoon I was having trouble pressing it. The lock button is very hard to press on if I want to lock or turn my phone off now. I have no idea how this happened to my phone or if it can be fixed at an Apple store because I don't think that I have a warranty on my phone. At the moment I am using assistive touch to lock the phone but it is extremely unconvienent for me since I am constantly texting and on social networking sites which result in me constantly locking my phone.