Q: Battery is draining fast and i'm not doing anything (11 hours to 5 hours in 20 minutes)

My mac used to last 12 hours when I have charged it, at decent brightness and was able to watch shows without significant battery change. Now 4 months later, my battery goes from 100% to 75% in an hour and has a maximum of 6 hours or less. What's going on?

yesterday I charged it to 100%, had one Skype call that lasted an hour and it dropped to 30%, left it to sleep for an hour and came back and it was 1% ???? This is ridiculous!