Why is my 2015 MacBook Air's battery draining quickly?

I got my MacBook Air in June of th year and everything has been great until lately. Today, for instance, I used my computer for maybe an hour in the morning and then put it on sleep mode with an almost full charge until this evening. Now, after only about an hour and a half of video streaming, it needs to be charged. What's the problem?

Also, the fan has been coming on every time I stream video as well. Could there be any relation?