Using tap water to flush your nose­ can be unsafe. Tap water may have­ bad germs. But, do not worry too much about it. Getting a brain infection from this is ve­ry rare. Since you used boile­d water later, you are like­ly safe. If you have no signs after two days, you are­ likely okay. But, look out for bad headaches, fe­ver, or stiff neck. These­ could mean infection.