The increase in the humidity setting may or may not be responsible for the cold symptoms you feel coming on.
Especially since the congestion and cold symptoms seemed to come on later this afternoon.
Here's the deal on the machine's cranking out humidity and the humidity itself sometimes causing nasal congestion.
Nasal mucosa are funny little bits of tissue and some nasal mucosa like a lot of moisture to keep them happy and some people have nasal mucosa that don't like added moisture.
When nasal mucosa get unhappy for any reason they all act the same way...from either too much moisture or not enough moisture...they swell up and give us the typical cold/allergy like symptoms that we all hate. They swell up, start to produce excess mucous, itch, etc.
We can of course simply have the bad luck to catch a cold. So if you have the other usual symptoms of a cold then maybe you just got some bad luck.
Did the change from the default humidity setting of 4 to 6 maybe cause the congestion? Maybe..maybe not.
Humidity needs are highly personal and each person needs whatever it takes to keep their nasal mucosa happy.
So if the setting of 4 seemed to feel a bit too dry...and if you don't feel like you are catching a real cold then maybe that setting of 6 is just a little too much humidity. Play with the humidity settings until you find something that works for you.
My nasal mucosa would be happiest if I simply snorted water.

When I forget to top off the tank and it goes dry during the night my nasal mucosa give me a real rude reminder as to why I shouldn't do that. I use the setting of 6 myself and at times have considered moving it up even more but then I would use up all the water too soon. It barely lasts 7 hours as it is.
Ambient humidity will play a big part in what your nose is used to. My ambient humidity where I live...not dry at all...40 to 50% and I still need pretty much maximum humidity or my nasal mucosa give me fits.
I may have to RISE but I refuse to SHINE.