I wonder if yeast infection is fungus?

Yes, it is.

I'v been taking clindamycin 150 mg t2 capsules three times a dayfor a tooth infection.
as a result I have a yeast infection that urination very painful. Is there
a topical cream that i can use ?

Are you sure it is a yeast infection? Don't try to make diagnosis by yourself - let your health care professional examine your urinary tract and make lab tests to estimate exact problem. As for your question about the cream, usually doctors prescribe medicines like Lamisil or Mycelex-G to treat yeast infections. Best wishes, support.

cause of fungus?
why is it infect other area of the body?

Fungus may be transmitted by touching skin affected by fungus. So if you have fungus on nails and touch other person, he or she may have fungus, too. That is why it is necessary to treat fungus. Good luck and be healthy!