Thanks for your question.
Without seeing your problem, it's difficult to tell with any certainty what the problem is. However, there are a couple of things that come to mind. The first is a yeast infection of tongue which is can easily be treated. However, the issue here is not the infection itself, but why you have it. Though it can occur in normal healthy individuals, yeast infections, or oral candidiasis, occurs more commonly in patients with an underlying medical problem such as diabetes, diseases that affect the immune system, malnutrition and a few others.
Another thought is that this could be a pigmented lesion of the tongue, which would require further investigation. If your problem has only been there a week or two, then a pigmented lesion of the tongue is unlikely. However, if this has been present for many months, then a tongue lesion (or growth) is a possibility.
In both scenarios above, the bottom line is you need to see your physician. He will ask many questions and examine your tongue, oral cavity, nose, neck and chest. Afterwards, he will be in a better position to say what the problem is, and what needs to be done. He may request some blood work or other studies.
I hope this has been helpful.
Martin Cane, M.D.