Not enough fluid. When you get up quickly, many times the blood in your feet take time to get to the heart. So, you can feel faint for a bit until the blood goes to your heart and then your brain. Add heat, and it can be worse because you are probably dehydrated. So, if this happens, make sure you have enough fluid. If it is persistent, see your doctor.
Head has less blood. A person who gets overheated will automatically dilate (widen) the blood vessels in his skin so that more blood will flow to the skin. His body does this in hopes of cooling him down, just like the elephant who sends more blood to his ears to cool his body down. The person, however, feels lightheaded and may faint because there is not enough blood flowing his brain (too much blood is in the skin).
Low blood flow to... Transient low blood flow to brain makes you feel faint. In hot atmosphere, blood vessels in extremities are dilated resulting in peripheral pooling of blood. Getting up quick further compromises blood flow to brain.