it started three after i bought the phone.when i charge it to 100%,after making 2 or 3 calls or checking watsup messages the battery will just drain to about 70% and the phone heats up and if i continue to use it, indicates low battery within 10mins with 5% remaining and go off.when i switch it on again the battery will increase to about 80% and after about 15% the phone is off due to low battery.my wifi and bluetooth are off.i have changed the battery but still the problem is still persisting. please help me.what should i do?