[Gill]

The only reasons I can think of that somebody might have told you it was bad to do cardio while training for mass are:

1) You burn calories. Any calories you burn while doing cardio are calories subtracted from your surplus (hypercaloric dieting).

2) Cardio if done improperly, induces catabolism which burns muscle. As long as you are eating something before, after or both then you'll be fine.

Personally, I'd continue to do your cardio. Doing so ensures you pick up as little fat as possible while building up your LBM.