Galaxy S 2 battery won't charge while being on

First of all, LOVE the Galaxy s2, even though it has some problems - which phone doesnt, right?

I've had the Samsung Galaxy S2 for 2 months. It worked fine. Since a week my phone won't charge the battery while being on. I would charge my battery almost every night. At night almost empty, I wake up and its full. Now a week ago I woke up and the battery had only 3 % left and it was VERY warm. Unplugged it, tried it again. Turned phone on and off... Nothing worked. When my phone died, I left it to cool and then charged it again in it's off state for a few hours. I turned it on and the battery was full again!

I use the original charger, I haven't changed anything since having the phone.

So now I can only charge my phone while it's off.. Not ideal...

Has anyone had the same problem?? Any advise is welcome!

Thanks!