Two main causes of this are ill conditioning and eating before you practice. Never eat less than two hours before practice no matter what shape you're in, you WILL throw up. I usually don't eat for about 3-4. The other part is just conditioning and there's no way to fix it except just keep going and eventually it'll stop once your body gets used to the pace. No reason to be afraid of it or ashamed because of it, it happens to everyone man. Best of luck!