What Causes Extreme Fatigue During Pregnancy And During Menstruation??

Hi, may I answer your health queries right now ? Please type your query here... During my pregnancy I got extremely weak in my arms and neck to where I couldn't dress myself, do my own hair, drive my car, etc. Now that I'm not pregnant anymore, I still get that weakness but only during my menstrual cycle and not to the extreme as when I was pregnant. What's going on?