when to take antibiotics?

Advice please...

I have drop in peak flow, had 2 attacks this week (not needed ambulance as extra steroid inhalers abated symptoms eventually). However, today I feel absolutely exhausted, have had some chest discomfort and sputum. So - when do you know where to draw the line and take antibiotics? I don't think this incessant rain has helped quite frankly... What indicators do you look for to start on antibiotics? Thank you.