Mike,
Thrush is a fungal infection caused by, candida albicans, a fungus that is present in most individuals that only causes a problem when it has an opportunity to take over and area. Thrush usually appears as a whitish coating that can be wiped off with a gauze. It will come back quickly if not treated. It can cause pian and othe problems but is fairly easy to eliminate once it is recognized.
Mouth sores or, apthous ulcers, have a variety of causes; weakened immune system, trauma, damaged tissue, and dry mouth are a few of the most common causes. They can be difficult to treat but usually are manageable. They appear as a whitish to white yellow circular area with a very red border. They can be solitary or in groups ranging from small dots to as large as 2 cm in diameter.

You did not mention if the lesion under your tongue was sore.

E-mail me if you want any more info.

Cheers,
Mike