Don't be alarmed. That ruby-red flush is just a sign that you're working hard and building up heat. When your temperature begins to rise, your smart little body perspires to keep itself cool, but it also dilates the blood vessels in your skin to reduce your body temperature (aka vasodilation). This circulates blood to your skin's surface so that heat can radiate off of it. So have no fear — your face turns bright red because warm, oxygenated blood rushes to it as a way to prevent you from overheating. This is a good thing and nothing to be worried about.
To learn when a flushed rosy face might be an indication of a problem, read more.
If your red face is coupled with symptoms of feeling exhausted or dizzy, sweating more than normal, or nausea, then it could be a sign of heat exhaustion. You're especially at risk in this hot Summer weather, so if you're experiencing all these symptoms, stop exercising ASAP, get out of the sun, loosen up tight clothing (or remove it all together), and drink plenty of cool water.
Tell me, does your face get especially red when you exercise?