Madonna's Re-Invention Tour, her first in three years, is getting off to a rocky start. On only its second date, the 45-year-old singer has already postponed one show due to illness. On Tuesday, a day after the tour kicked off at Los Angeles' Great Western Forum, a statement posted on her website read: ''Tonight's show at the L.A. Forum is postponed until tomorrow night. Madonna has the stomach flu and was ordered by her doctor to rest this evening. She will be back at 100% and can't wait to entertain her fans!!!! If you have tickets for tonight's show, they will be honored tomorrow night.'' After the show rescheduled for Wednesday, Madonna is due to play one more night at the Forum on Thursday, then move on to Las Vegas on Saturday. The tour is scheduled to run through September, with 50 more dates slated in the U.S., England, and France.