Jennifer Lopez is "getting frustrated" with Casper Smart.

The 43-year-old singer and actress started dating the back-up dancer last year following her split from Marc Anthony but friends of the star say she is losing patience with her lover. "[Jennifer] is getting frustrated with him and she's increasingly short-tempered when she's around him. He's moved into her life - and a little too easily, too. I think her family is watching him closely," a source told People.

Jennifer's feelings are surprising considering last week her 25-year-old boyfriend threw a lavish bash for her 43rd birthday, on the yacht Utopia III, which is owned by Market America co-founders JR and Loren Ridinger.

However, friends don't believe the relationship will end soon as they think Jennifer, who has twins Max and Emme with Marc, will want to have a new love interest in her sights before she breaks up with Casper. "Jennifer is the kind of girl who doesn't like to be alone. Until she finds a replacement, I think, he's around," the source added.