I had this prob just this week.
I never did dress Mocha. One because I couldn't find nice dresses and two because I live in the tropics. But just this week I took both of them to the mountains for a vacation, and she developed the most horrid tangle under her armpits
I was so upset... She never had tangles before. If you see my dog Lulu you'll know why. I brush my dogs religiously. I tried to separate the mat with my fingers, but it just pulled and pulled and i couldn't cut it either.. she's very twitchy. She got hurt by that tangle
Now she's afraid of the combs and brushes... I am soooooo upset. All that work to get her get used to grooming, gone with a tangle