Maybe with such a big squooshy saddle you're sinking into it and that is how you're getting pressure on the tailbone.

http://www.serfas.com/comfort_saddle...tive_all.shtml

It seems counterintuitive, but a firmer saddle is more comfortable for most people, since it supports you correctly instead of trying to cushion you. Can you borrow a more traditional bike seat from someone and see if it feels better?

Nanci