Changed new battery yesterday- car died today

[cfx5]

hey everyone,

on sunday night i drove my 2003 x5 all day. At night i went to my gf house and let the car sit for about 20 min and i came out and the car wouldnt start. I was like great.....time for a new battery..i jump started it the next day..drove it 30min to batteries plus. I turned off the engine in the parking lot and tried to restart it just to see if it would start and it was dead...so i went in, got a new battery..had the guy changed out my old one and i went home. Next day, my mom borrows my car and when she came home she said that she barely made it home, that car was acting realy funny. I go outside and try to start it up and the battery is dead again...any ideas?