goodplasticsurgery claims that Paris Hilton might have got implants. (click on link for picture) I don't know...first of all I don't think she was very flat chested before. Small yes, but she's always had a little something.
Her boobs do look smaller some days than others, but I think that's normal with smaller breasts, weight fluctuation and "that time of the month" make a bigger difference. Also she is more tan in the second pic, so that creates the illusion of larger breasts.
I vote for no.