I keep having a problem with my left breast, where milk seems to build up into a lump which I can't shift. It is really painful and I'm worried it might turn into mastitis. Baby is now asleep for the night, and despite making her feed from that side for an hour before bedtime the lump is still there. Expressing doesn't seem to help either. This is the third time this week it has happened, but on those occasions the baby was able to shift the lump by feeding eventually. So strange, I have been BF for 5 months now, and other than getting engorged from time to time I have been fine. What could be causing this, and more to the point, what can I do to relieve the pressure?