Selena Gomez was hungover during her appearance on 'The Tonight Show with Jay Leno' on Tuesday evening (07.23.13).
The 'Spring Breakers' star celebrated reaching the milestone age on Monday (07.22.13) with friends at the Bagatelle restaurant in West Hollywood, California, before hitting Beacher's Madhouse club.
Selena was able to enjoy alcoholic drinks, including famous Tennessee whisky Jack Daniels, legally in the US for the first time and Leno wanted to know if she was suffering the after-effects of her big night.
He asked: "And how many of those (Jack Daniels) did you have?"
The brunette smiled and replied: "I don't remember."
Continuing to quiz her, he said: "Are you hungover today, was it a big party?"
To which Selena answered: "A little bit, it was fun though, it was good."
The former Disney star, who was promoting her debut studio album 'Stars Dance' on the show, is planning to continue partying to mark her 21st birthday with a "gypsy themed" bash over the weekend.
Selena - who was not joined by her on/off boyfriend Justin Bieber on Monday - revealed: "I'll have a birthday party this weekend but I had a dinner with my friends, the party's going to be kind of a gypsy theme, I'm excited.
"We're gonna have skirts and tattoos and belly dancers. It'll be kind of the whole thing, It'll be fun."