iPhone 4 vibrating for no reason and cutting sound when plugged in.

Hi, my iPhone 4 (which is only around 7-8 months old) has begun vibrating occassionally for no reason. It will do this even when left sat on a table without being touched and without any incoming posts, messages, calls, etc.

It has also begun vibrating when I plug it in to charge, which it never used to before. Not only this, but it will sometimes also cut the sound from the speakers a few seconds after being plugged in to charge - and can only be listened to through the headphones.

I have not set it to do any of this, and can't find any way of stopping it doing this through the settings.

Can someone explain to me what's happening?

Do I ALREADY have to replace a phone I bought less than a year ago?

PS I can't remember if this is an iPhone 4 or 4S, and the packaging is in the attic. If it makes a difference I can try and get it out.