My Budgie Laid An Egg But I Don't Think The Pair Mated Yet

hey guys, just a bit confused, cos i got a new budgie today (female) and she laid an egg maybe an hour after i got her. im kinda thinking that she mated with another male at the breeder we got her from and laid an egg from him. or maybe (i dunno if this is a fact) but could it just be a dummy egg? also when we got her she had a bulge near her butt but after the egg was laid it disappeared. would be glad for any type of advice on what to do or what possibly happened.

Thanks in advance.