My periods have changed since being on anti-depressants - I think I bleed the same amount but in a much shorter space of time, so a couple of heavy days then very little - whereas before it would last 4-5 days. I've never asked a doctor about it, just kind of accepted it. But as others say, if pads are getting soaked very quickly, it is definitely worth asking your gp about it as there may be a reason for it.