Airlock activity doesn't mean anything. I haven't seen bubbles at all in several batches, even though they completely fermented. You probably bottled too soon and now you have gushers. I've never had an infection, but from what I understand, most true infections give a sour taste. When I've had gushers before, the CO2 seems to all come from the bottom of the bottle, and push all the yeast and sediment up. That's probably what you are tasting and why some have more of an off-flavor than others.