There should be no issue about going to a different cabin in order to use the larger lavatory (I always have trouble saying "accessible lavatory", since the airline definition of "accessible" is so wildly off the real world mark!).

For some discussion of alternatives to using the lavatories, see these threads on CareCure (not all female-specific, I'm afraid):

http://sci.rutgers.edu/forum/showthr...g-in-the-plane!

http://sci.rutgers.edu/forum/showthr...ile-on-a-plane

http://sci.rutgers.edu/forum/showthr...st-time-flying

http://sci.rutgers.edu/forum/showthr...1-foley-travel