If you have conceived, the ball of cells known as the blastocyst that will eventually form the fetus will now be preparing to embed in the lining of your uterus, and the placenta will be starting to form.
Before this happens, however, there is another important change going on. After you ovulate, the empty ovarian follicle develops into a structure called the corpus luteum (which means, literally, "yellow body"). This small, fluid-filled sac becomes increasingly "vascular," developing blood vessels and beginning to produce the hormone progesterone. This is required to create mucus to allow your fertilized egg to survive, and build up the lining of your uterus, in which the blastocyst will soon imbed (see You are 3 Weeks Exactly).
The corpus luteum also produces a little estrogen. By about 8-12 weeks of pregnancy, your placenta will take over the production of progesterone, but the corpus luteum continues to play a small role in hormone production until about six months, when it usually shrinks away.