First, if the battery has begun to expand, do NOT attempt to put it back in the computer. It can interfere with normal operation of the keyboard and trackpad, and in extreme cases damage the computer.
Batteries generally only start to expand after they have aged quite a bit. If it were new this would be very unlikely. Also, if they replaced that battery they would have charged you for a replacement, as batteries are considered consumable items generally not covered by warranty under normal use.
Best suggestion is to just go buy a new battery.
Best of luck.
Thanks! My computer is 2 years old, the battery was perfect until I took it to the Mac store. I took it there to add more RAM, when I got back home I noticed sth strange with the battery. Then, after a week I couldn't put it in again. That's why it is sth suspicious
Do I have any way to check if they have changed it?