Severe bronchitis has forced rock singer Stevie Nicks to postpone concerts last night (Aug. 21) and tonight at the Universal Amphitheatre in Los Angeles. The former Fleetwood Mac singer experienced some respiratory problems last month as she embarked on the tour in support of her latest Reprise album "Trouble in Shangri-La Tour," according to her spokesperson.

As the tour went on, Nick's voice grew more hoarse. After a concert Friday in Santa Barbara, Calif., "[it became] clear that she's not going to get better until she gets off the road and rests for a while," the spokesperson said.

The ailment caused Nicks to cancel another pair of California shows last week, Aug. 15 in Mountain View and the next night in Concord. Those shows have been rescheduled for Sept. 30 and Oct. 1, respectively. Rescheduled dates for the Los Angeles shows are pending.

Nicks will undergo tests and treatment in Los Angeles. Her next scheduled shows come Friday in San Diego and Saturday in Las Vegas. In the meantime, the single "Planets of the Universe" continues to perform well; it's at No. 11 on Billboard's Hot Dance Music: Club Play chart and No. 14 on Billboard's Hot Dance Music: Maxi-Singles Sales tally.

The artist has also unveiled details of a Dec. 6 benefit concert at the America West Arena in Phoenix, proceeds from which will benefit the Arizona Heart Institute Foundation. At last year's event, Nicks was joined by her Fleetwood Mac colleague Lindsey Buckingham, Sheryl Crow, Chris Isaak, and Don Henley. Visit Nicks' official Web site for more information.