[StGeorge]

She actually tried to warn Caoimhe about the Shabby issue weeks ago, so it not exactly new or going behind her back. The fact she made comments in the DR, whilst being as she put it "angry" , is not surprising.
But going back to the OPs question..why then confess??

We the public sometimes forget that these HMs are constantly thinking about how they are perceived on the outside. Ife would reflect on what she had said and assume it could be looked at as being rather snidey if shown, seeing as she then became chatty with Caoimhe again. And so to look like she is an honest and up front sort of person, she confesses.
But it backfired as Caoimhe got the hump and went around recruiting her troops.
Josie in particular has become her main gossip ally (big mistake by Jos) and of course Josies lapdog JJ goes along with her.

Its interesting that unpopular Nathan has now switched allegiance but it could be too late, and popular Josie could be cosying up to Miss unpopular which could jeapodise her standing over the next week.