My friend had blood lactate 3.7, after drinking water feels cold chest cavity followed by full body prickles. er found nothing. what could it be?

Well, the most: obvious causes of lactic acidosis--severe bleeding, shock, infection, heart failure, CO poisoning, pulmonary embolism--would have been detected by ER. It can be associated with dehydration, severe anemia/leukemia or liver disease (probably also checked by ER), extremely strenuous exercise or overheating, alcohol/antifreeze poisoning, some medicines: INH, metformin. Needs follow-up by his Doc