I had morphine while bf, and I don't think you are strictly meant to, but don't fret
definitely no decongestants though
try inhaling steam (with menthol if you have any) - hang over a bowl of boiling water with a towel over your head, this will clear your sinuses
to shorten the cold take elder berry extract from health food shops - also can be used as preventative if you feel a cold coming on, or are around lots of ill people