we've been going through this with Natalie for over 2 months now! So after my GP brushing it off for 6 weeks

I got us referred to an allergy specialist - I thought that seeing as the congestion started at about the same time her eczema showed up, it might be allergic rhinitis (like hayfever).....however the specialist last week thinks it might be enlarged adenoids which are preventing her nose (and ears) from draining properly. so off to the ENT specialist next week!
So this may be no help at all, but I just wanted to say make sure you get it checked out with your GP, and if they brush it off as a cold, keep pushing. We've had 2 months of insanely broken sleep (some nights she's so blocked up she will only sleep on us), and has now started having mild sleep apnoea at night which is a bit scary!
Also wanted to add that before we got to this point we tried karvol, baby vicks, humidifier (with and without oils in the water), tea tree oil in the bath, elevated mattress, sleeping on her tummy (which seems to help), nasal drops, the horrible sucky-nose thing and antihistamines (prescribed by GP). What does seem to help is for her to be upright as much as possible during the awake times, and trying to wipe/massage the snot out of her nose before sleeps.
good luck - it sounds exactly like what we're going through