My periods are due in 1-3 days.When I urinate,I see one or two small clouds of white discharge.They seem to dissolve in the urine and disappear in few seconds.This is happening from yesterday.I don't have any itching or pain.Is this an indication of the onset of my period or could I be pregnant?

Sounds like ovulation discharge and most likely your period will come on soon.

I don't find any discharge when I wipe with a toilet paper.Its only in the urine.Could this be urinary infection?

It very well could be a UTI. Any backaches or burning on urination? Sometimes there are no symptoms with a UTI. Better to have it checked out by your doctor to be sure because UTIs need to be treated with antibiotics.

No.I don't have any burning or backaches.
But I'm urinating frequently.

You should probably get to the doctor to get this checked out. It sounds as though it is a UTI. Please make an appointment.