For a lot of women who had previously been full of energy, suddenly find they become very tired in early pregnancy (and towards the end), many experience morning sickness and breast tenderness, also headaches are very common as your blood volume expands. The symptoms do improve and usually by about 12-14 weeks most have been resolved.
This article explains why.
"The first several weeks of pregnancy can be strange to say the least. At times you might feel completely normal; your jeans still fit, and no one can tell you're pregnant. Other times you might feel like you're moving in slow motion and about to fall asleep on your feet. And although the side effects of early pregnancy may seem freaky, rest assured that what you're feeling is almost certainly normal. How many of these classic signs and symptoms of early pregnancy do you have?