A strength loss of that much is completely normal, at least it is for me. I'm sure it varies among women. That article kpsnap mentioned is a very good one although the times when I am strongest/weakest are different than the timeframes mentioned in the article. Again, I'm sure it varies among women. What I learned is to not get down on myself when I find I can't lift as much as I did the prior week. Drop the weight and do more reps. Or, ditch the lift altogether and work on weak areas such as doing rack pulls or stiff-leg deads instead of doing deadlifts. The strength will come back and often comes back more than what it was. Just don't derail yourself mentally when it does happen. Realize it does happen and have a Plan B for when it does.