Originally Posted by Deaky
...because I was fairly close behind the singer and because the stage was so small it was thought that my bass was being picked up by the singers mic and therefore picking up this sound effect!
Is that a fairly common occurrence
Yes, very common. The effect wasn't coming from your amp though. It was just your bass notes from your amp were going into his mic, and through the effects from there, through the PA.
To avoid it, make sure your speaker is pointed away from the mic.