I wonder if someone here can help me

This morning (I'm in SE PA) a little bird--goldfinch size--landed on a small tree outside my window. It was nondescript grayish, with gray/black stripes. But it had a big bright yellow patch on its butt! This was not a goldfinch; I know what they look like in early spring before they get their yellow. It was shy and flew when I tried to get a closer look. This guy looked like somebody had painted a big yellow spot on his butt--not his tail, his/her backside. Roger Tory Peterson was no help; I'd hoped it was a Confusing Spring Warbler, but I couldn't find a picture that even remotely resembled it. Any ideas? It has not returned today, and we have heavy bird traffic here.