Low Frequencies always can mess everything when enters in Stationary behaviour.
All closed spaces can cause Stationary Waves. If in good sounding ambients this frequencies are best distributed and doesn't bothers you, in problematical rooms just some frequencies build up (annoingly twisting room's frequencies response curve towards mud low-mids).
As low frequencies have bigger wavelenght, they tends to be more problematic (since they also build up a complete Harmonic Series, wich means that it causes ressonances in partials of the main problematic frequency). Don't forget that lows are omnidirectional and equaly spread all over the place. While lows shows up behind PA as strong as they roar in front of Speakers, highs - that moves forward (away from you) - sounds weak at the stage. I believe that this is what was bothering her... Specially if drummer's monitor was pumping the bass
A room's stationary wave occurs when the distance between its parallel walls is half of a size of its wavelenght. In fact, these distances do defines wich frequencies will build up. So it is utterly important that distances between walls in a room never relate. The worst case would be a cubic (or spherical) room, when the reinforced frequency will be the same on all axis. But it is bad when a dimension is half or double the other either.
When this happens, not only omnidirectional lows are overwhelming, but even some specific notes can be tottaly up front (even if you play them short and softly). Stages like that are totaly exasperating to play on and this bomb notes can be very distractive (and distructive) to your performance.
