Justin began dating Britney in 1999 and broke up in 2002, when Justin was 21-years-old. Given his reference to how young he was. It seems like she may just be the girl embarrassed by his mother's interruption.
This of course after Justin admitted the two were in fact intimate while they were dating.
(As Britney said: when you find that right person you just know,)(I will love to be with him forever i would, i would ", )