The generic name for sores in the mouth or tongue is stomatitis, and there are many potential causes of stomatitis. The best first step for you would be to go see your primary care doctor
, who will be able to help you figure out what is going on exactly.
One very likely possibility, based on your description of the sores, is that these are canker sores. Canker sores are small painful ulcers that occur in the mouth and that do not have any clear source, although they are often linked to stress and tend to flare up when you are fatigued and under the weather. Canker sores can be treated with steroids to reduce the inflammation and encourage healing and with several topical medicines to numb the pain.
Another possibility is that you have a viral infection of the mouth, such as herpes. These infections are also very common and they cause multiple small sores which tend to be very painful. Fortunately, these ulcers do go away on their own, but often medications are needed to help with the pain while they are healing.
Finally, there is a rare autoimmune condition called Sjogren's disease that causes both mouth sores and also severe dry mouth; this is something that your doctor can consider if the more common conditions do not seem to be the cause and if the dry mouth symptoms are severe or persistent.