What does your battery status say when you click on the battery icon new the clock?
Mine was saying "Service Battery" for a few months and would die very quickly, then towards the end of the "Service Battery" when you unplugged it just died instantly as if there was no battery at all. Then when "Replace Now" came in it chopped the speed in half it and it all went to crap.
Yeah I replaced it myself - it's super easy. You just take the unscrew the bottom cover with a tiny star screwdriver, and then once you get the cover off it's just 3 screws holding the battery in and then you pull the tab to take it out.
Oh hm no I haven't gotten any messages like that yet. How old is your Mac?