My lil one was breastfed and had congestion like you describe periodically - he didn't act sick, no fever, etc. I suppose I'll never know for certain, but I suspect most of the time it was teething related. That, or perhaps a very, very mild cold. I wouldn't worry too much, but do look out for signs that it may be getting more serious (fever, lots of green snot, green goop in the eyes, baby not acting like himself, lethargic/clingy, etc.). If it seems like it's getting worse, I'd consider getting him checked out - otherwise, it's probably fine. I can remember my lil one having that fairly often as an infant. Good luck!
eta: cold mist humidifier, saline spray (lil noses), and bulb syringe (for sucking out boogers) have been lifesavers for us! Also, as an alternative to the saline spray, just keeping baby in a bathroom with hot shower running - the steam will do the trick.
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