I'm on effexor (venlafaxine) and recently on amoxil for broncitis... feeling quite flat. can amoxil cancel out the effexor (venlafaxine) ?

Unlikely: There have not been interactions between these two drugs reported. Although that does not exclude the possibility of idiosyncratic reactions, your depression may be worsened either by the fact that you have been ill with bronchitis (which may not need Amoxicillin for therapy), or for unrelated reasons. Suggest you contact your therapist and discuss the alteration in mood at length.

Amoxicillin tired: In the small print under rare side effects, Amoxicillin can cause somnolence or tiredness. It also can cause insomnia and confusion. Effexor (venlafaxine) over time can actually turn and cause substantial tiredness. Agree that underlying medical problem can be causing it. The depression may as well have escaped the earlier provided benefit of the Effexor (venlafaxine) and need to have dose adjusted. Note if any new snoring.