When you exercise, your core body temperature rises from the work your muscles are doing. Blood is diverted to the skin (and you go a bit red) where body heat can be radiated away. Sweating helps this process by removing heat as it evaporates from your skin. If the ambient temperature and humidity is high, then the blood cooling process is slowed, sweat evaporates slower, and it becomes harder to the body to regulate it's core temperature. Also, while the blood is being cooled in the skin, it's not feeding vital oxygen to the muscles that you are working, meaning you will fatigue much faster. Counter this by working harder in shorter bursts, and take frequent breaks to allow your body to cool while you grab a drink.
The average person will sweat between 0.8 & 1.4 litres during an hour of exercise, that's about 1 ½ to 2 ½ pints in old money. You know that 500ml water bottle you have in your kit bag? In hot weather it's not going to be enough. As your body loses fluid, your blood volume decreases and it is able to carry less oxygen to your muscles, and when some of your blood is already going to the skin to regulate body heat, your muscles will start to ache and cramp up much faster if you are dehydrated. Make sure you drink plenty of water throughout the day and especially in the hour before you exercise so that you begin your workout well hydrated, and replace lost during your training fluid by taking regular drink breaks.
Through sweating you not only lose fluid but also electrolytes. Electrolytes are nutrients that carry electrical signals around the body, regulating heartbeat, triggering muscle contraction and transmission of nerve impulses amongst other things. An electrolyte deficiency in the body can slow your reactions, and cause nausea and muscle cramping. Don't believe the marketing hype of sugary 'ade' sports drinks, water is best for keeping you hydrated, and a healthy diet best for replacing nutrients.
Listen to your Body
If at any time during your training you feel nauseated, light headed, dizzy, cramping, or have an irregular/weak/unusually high heart beat, STOP.
There are no medals for putting yourself in hospital, and no-one will think you're a hero for collapsing in the middle of a training session. Take a break, lower your body temperature, take on fluids, fuel yourself correctly and get back to it when you are ready.