I was going to say what @gondwanalon said, it probably mostly has to do with your digestive system shuttng down while running, so oxygen filled blood is moving well in the muscles that are demanding it. Most people have a brief period after exercising, especially if they exercise in the heat, where they have no appetite, and then a short time later they are ravenous. Many people above mentioned staying hydrated, I agree, but I am betting you already do your best to stay hydrated.
You probably are pushing yourself a little too hard, as many have said above, and it is traumatic on your system.
Also, make sure you have some carbs before you exercise so you don’t deplete your glycogen stores completely.
I don’t know how hard you are running, but if your heart rate is going very fast you are probably leaving aerobic exercise for anoerobic.