IPhone 4 Lock Button (top button) is hard to press/almost stuck

I have had my iPhone for around 11 months now and have never damaged it in anyway. Until today the top button (lock button) has been working great, but all of a sudden it's really stiff and hard to press compared to what it was. It still works, but that's after it's been pressed a few times with pressure.

Does anyone know what has happened to it?
And what I could do to fix it without buying a new phone/taking it to be repaired/replaced?

The exact same thing has happened to me, I had my iPhone 5 for around 7 months, took great care of it and then the lock button stopped working for no reason, the same goes for my iPhone 5s, it has also began happening recently to my sisters iPhone 5, I'm not entirely sure what it may be, but it could possibly be worn out from getting pressed.

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