Rihanna has voiced her frustration after she was forced to cancel two shows in Texas this week (15-16Apr13) due to illness.

The We Found Love hitmaker pulled out of her concert in Houston on Monday and in Dallas the following night to rest and recover from an undisclosed condition.

The pop star was photographed leaving a doctor's office in Beverly Hills, California on Tuesday (16Apr13), and she took to her Instagram account to express her disappointment about the axed gigs.

Rihanna writes, "I was sick since (my 11 April concert in) San Diego! I kept working because I thought two days off after the Vegas show (on 12 April) would be enough time to recover and I f**ked up, those were two days off from hell and today is not different.

"I'm sad and disappointed about it! I had all kinds of plans!"

Rihanna has vowed to reschedule the concerts. She is next due onstage in Tampa, Florida, on Friday (19Apr13).