Double Bass On the brink of Feedback

[Audiophage]

I've been playing in a pit band for a musical at my college that ran for the last 3 days. For some reason, on the second day of the performances, something was sympathetically setting off my D string when I wasn't playing and muting the strings. It was completely gone on the first and third days. I hadn't changed any of my levels since the tech rehearsal days, and I don't think anyone else had. I'm rather new to amplifying an upright bass, and I don't even know what was causing this. There may have been some other physical properties at work. Either way, does anyone have any ideas of what was causing this beyond what I've said?