Women gain muscle as fast as men. It turns out that the "lack of testosterone inhibits muscle growth" thing is wrong. In fact, neither testosterone nor growth hormone in physiological doses promote muscle growth:

http://www.medicalnewstoday.com/releases/246637.php

http://jap.physiology.org/content/112/11/1805

http://www.springerlink.com/content/228h54586751g19l/?MUD=MP

http://www.springerlink.com/content/r866q8m835680400/

So, women don't get as big as men because, on average, they have smaller muscles to begin with, not because they lack testosterone.

Good news for bodybuilders, though. Pharmacological doses of growth-promoting hormones do promote muscular hypertrophy.

To answer your question, it's impossible to say for sure that your weight gain is all lean mass without measurement, but it's not unrealistic.

If you're really concerned about it, have your coach break out the calipers. Measure your body fat now and then in another month or two.

2lbs of muscle is a lot for a woman to gain in 5 weeks, but water fluctuations, time of day you weigh yourself, etc. can exaggerate the scale. Go by how you feel, how your clothes fit, and objective measures of body fat (tape measure, calipers, etc.)

take pictures regularly it will show you a visual progression, also i have only lost 15lbs since i started xfit/paleo but inches is a different story

How do your clothes fit? If you have trouble telling by looking in the mirror (cause you see yourself every day) have your trainer take pictures of you, so you can see if you are gaining muscle.

In my opinion, yes, you are gaining muscle. When I worked with a trainer, my arms gained muscle soooo fast, within a week, they were getting too buff!

You will hate my answer but you may still have hope if it doesn't apply to you. I am fairly muscular to begin with but I lacked any upper-body strength and no abs. Just legs. I am not skinny. I'm large-framed but no longer fat. I have been doing core fitness exercises since January. Took me several months to see results. I could feel them before that, but seeing the muscle definition has taken a very long time and there's still a long way to go.

It depends a little on body type (a mesomorph puts muscle on much easier than the other body types).

Also, how heavy are you lifting? The best results are gained by going pretty heavy (heavy enough that you can only do 6-8 reps...lighter days and assistance work is fine and helps also, but you will probably see most gain from throwing in some really heavy rounds).

Finally, give it a little time. 12 weeks is a good measurement period (it's a minimal amount of time if you consider how long your life is). Go out and get a good measurement - we use a bod pod around here...$25 or $30 to get your body fat tested and it's one of the more accurate methods of body fat testing. Many Universities offer DXA for pretty cheap too. Calipers are only as good as the person using them. Body fat scales are crap. Return to the same place after 12 weeks for a body fat re-test.

You should see noticeable results after 12 weeks...if not, get a new trainer.

Go by body measurements in total inches rather than the scale. Add up the measurements from waist, hips, both biceps, and both thighs. Measure once a week. If its going down, no matter what the scale says, you're losing fat.