Lactose is the sugar found in milk. It is a disaccharide which is made up of glucose and galactose. When you are lactose intolerant, your body does not produce the enzyme lactase which digests this sugar. Hence the milk stays undigested in the can cause series of problems ranging from diarrhoea, rumbling, nausea, abdominal pain etc.
You should take not that these symptoms could be caused by other problems but If you only experience those symptoms only after taking milk and milk containing products, then you are likely to be lactose intolerant. However, you can take further test at the hospital to ascertain that fact. There, you blood sample will be tested for sugar after consuming milk. If there isn't any rise in sugar level, you are likely lactose intolerant.