Is it necessary to change clothes upon feeling the discharge of drops of urine after urinating, which leave a stain and cause doubt about ritual purity?

If you are certain or highly assume that a drop of urine has descended or that there is a changed color from the effect of urine, then you must perform istinja' again and wash the place of impurity on the garment, then perform wudu' and pray in it. However, if it is merely a doubt, the basic principle is that no drops have exited and that the clothes are pure. This principle is not nullified by doubt, and the change in the color of the clothes may be due to sweat.