The 'Radioactive' singer and the reality star ended their romance in December due to her hectic work schedule, but the split turned sour after the 'Keeping Up with the Kardashians' star accused her of cheating on him ''20 dudes'' while they were together.
Rita, 22, has now spoken out to address his accusations - which he made publicly on Twitter - and insists she never even classed herself as Rob's girlfriend.
In an interview with Australian newspaper The Sunday Telegraph's Sunday Style magazine, she said: ''I never thought it was actually a relationship, in all honesty. I never mentally defined it as 'boyfriend, girlfriend'. When I split up with him, I said, 'It's because I'm never there, I don't know how to do it.' That's all I said, then ... the rest happened. He obviously felt that creating myths was the way forward.
''But I don't think about it. I don't vocalise things that don't deserve attention ... In my world, things like that just aren't important.''
Even though her last romance ended badly, Rita hasn't been put off men and has revealed she has a crush on 'Twilight Saga' heartthrob Robert Pattinson.
She said: ''It's a terrible feeling, being lonely ... I'm still a girl. I'd love a boyfriend. But it's hard when they're not in the industry. I wouldn't care what he did as long as he made me laugh. I love laughing.''
Speaking about her crush on Pattinson, she added: ''I never even knew he was British. I thought he was American!''