Seems like Rihanna's health is once again a problem for her Diamonds tour.

The 25-year-old star has yet again failed her fans and cancelled another concert in the tour, making this one her third cancellation.

An unidentified illness continues to plague her, forcing her to take her Houston show off her tour, which was supposed to take place Monday at the Toyota Center. Unfortunately, fans were reportedly notified just hours before the concert.

The Bajan singer's concert promoter Live Nation issued a statement saying Rihanna was unable to perform "as a result of illness," but did not further explain what was wrong with the star. The mysterious illness forced fans to retain their tickets for a rescheduled date, which is yet to be announced.

The R&B songstress' next concert is set in Dallas and is still slated to proceed as planned.

To address her fans' woes, the Grammy-winner took to Twitter, telling her fans that she had been feeling sick since last week after her San Diego performance and apologized, saying, "This is the hardest thing for me to deal with!"

Houston is the latest stop in the singer's Diamonds World Tour which began in March, and this is not the first time she has cancelled her shows. The last two stops Riri cancelled were in Boston and Baltimore, allegedly due to laryngitis, and her doctor has advised her to rest her voice and even make a lifestyle change. Reports claim that this throat problem has once again returned--the reason for her recent tour cancellation.

Successful stints of Rihanna so far include several cities across the US. Among these are Buffalo, Philadelphia, Montreal, Toronto, Detroit, Chicago, and LA.

Meanwhile, despite her current condition and situation, Riri still was able to express her condolences and sympathies to the victims of the Boston Marathon explosion, tweeting, "Heartbreaking. #PRAY4BOSTON."