If the sound was ok to start off with and you did not turn the volume up, be honest did you, then the my moneys on the PA
As gilmour says when working with a PA let it do the work, play at a sensible volume.
In small venue it is very easy for backline to over power the main PA.
Also bass frequency waves are quite long so standing in front of a speaker is not always a good place to judge volume.
IMHO, unless the there is a real need, or the stage is large and you have great foldback there is no need to put any thing other then vocals though the monitors.