In order to gain muscle strength and hypertrophy the idea is to overload the muscles more than they are adapted to lifting. Which for many women is much heavier than we give ourselves credit for.
Below is a link to the Ask the Experts Response as to how much weight one should lift to get the results they desire.
How long before lifting did you eat? Strength training is an anaerobic activity which means that the body's primary fuel is glycogen, so you want to make sure that you refuel ASAP after your workout with some carbs and a little protein. If you continue to experience this issue or if you develop other symptoms, you may want to talk with your doctor about this.
I hope this helps!
Coach Nancy www.sparkpeople.com/community/ask_the_expe