Help Samsung Galaxy S3 Help!!

Well I'm new to this, I have a Samsung Galaxy S3, about 3 days ago I started noticing that my phone would not charge right. Two nights ago, I charged it while the phone was off, in the morning the phone said it had 100%, on the charging icon it shows when the phone is off and charging, well I took the charger off, turned the phone on to find it at 60% why did this happen? Last night I charged it, this morning woke up to find it at 100% said the icon, but then, like 20 seconds after taking the charger off it went down to 40%.. how is this possible? Please someone help. Tips: I mess around with my phone when the charger is on. :It is used. :Sometimes the charger won't charge I have to move the charger around until my phone beeps