[artemisyna]

Another possibility: Perhaps one of y'all ended up bringing up the topic of cereals (and those cereals in specific) because of a new ad campaign one of y'all saw.

You hadn't seen it yet, but lo and behold, you did later.

[thiht]

I'm not native so sorry for the weird question, but is "y'all" a common idiom? It sounds weird, is it something I could use or does it imply something particular?

[t-writescode]

English does not have a distinct, second-person plural. There are several regional attempts to make one.

Y’all is the Southern second-person plural, a contraction of ‘you all’ (you, second-person; all, suggesting more than one)

[omosubi]

this is becoming common in the rest of the US as well because of the lack of second person plural. they call it y'all sprawl. I hear it frequently here in Chicago.

[itronitron]

y'all is a common idiom in the southeastern US as well as Texas. It is a contraction of 'you all' and can mean an individual or one or more members of a group.

all y'all by comparison is generally agreed to mean everyone in the group and can be a replacement for the phrase every last one of you