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!
Try factory reset. My phone had this issue as well. After a factory reset it worked fine. But be sure you back up all the important data first.