I have itchy and frequent urinate for a week. Now I have sharp pain in left lower back. Does yeast infection cause the pain or sth else?

Something else. Yeast infection, limited to the vulva and vagina, is not likely to cause back pain. You may have an associated uti, but the history you provide is too sparse and you also need to be examined. See a doctor soon, and hope you are better in a short time thereafter.

Related Questions

Recently had uti. Now have white sediment & chemical-smelling urine & burning during sex. My partner says he now has itchy genitalia. Yeast infection?

Antibiotic use? If you took or are taking an antibiotic, then this may be normal. A yeast infection is not uncommon for the female that is being treated but not the male.