I used to be highly introverted. Now I can happily operate in crowds or by myself--I actually require doses of each to keep an even keel. Too long without either one and I get anxious, which kicks in the OCD.
I'm not convinced that preferring solitude or small groups is rooted in any disorder. Some folks just prefer to interact with others in a fashion that doesn't support large groups. Perfectly social people that have to regulate their interactions and make certain they get enough time alone to reduce the sensory input. The anxiety that surfaces from too much stimulation is not, I think, due to any underlying disorder, it's just a sign of overstimulation.