Great find, and within the expected window of Sulphur-bellied Flycatchers showing up out of range in North America.
I found the first state record of Sulphur-bellied Flycatcher at Adobe Creek Reservoir (Bent County) on September 18th, 2002. It was the first record for the interior of the continent, although there had been records during the same window of time in Louisiana and New England. Here is a picture I took on September 18, 2002, of the Colorado Bird. This was a bird I didn't expect to be seen again in Colorado. Many Colorado birders of an earlier generation got to see this bird.
Duane Nelson
Las Animas, Bent County, CO