The singer is believed to have used to naughty word in a post to her friend, US rapper Katt, after he compared her to a cartoon dog.
According to The Sun, he wrote, 'Watchin clifford the big red dog =) somehow im reminded of @Rihanna idk (I don't know) why tho.'
In retaliation, she Tweeted, 'LMAO (Laughing my off), Katt u lil c***.'
The shocking language came just hours after the 22-year-old star had reached the 3 million mark for followers on the website.
One angry fan told the newspaper, 'Rihanna is a role model to millions of young, impressionable kids.
'She shouldn't be using that kind of language - especially when she knows how many people follow her.'