I have the EXACT same problem I called apple and here's what's going on: My phones battery is like done. The battery's condition is ONLY 30% so when Ibplugged it in over night, ok ok right now it's 8:49am but my battery is just 18%! Here are you options because the battery isn't going to get better.
•Get the battery replaced
• Buy a new phone or upgrade to a higer generation
Hope everything turns out well for you. Hopefully same with me.