After I rinse with NeilMed Sinus Rinse I experience a runny nose and a sinus headache. Why could this be?

Thanks for your question. If you experience a runny nose after using the sinus rinse, consider blowing gently while keeping your head tilted to the side. This will allow the nasal secretions to drain out as much as possible and hopefully you will not have the runny nose symptoms. As far as sinus headaches, it is not a common thing but if your solution does not have the correct concentration it is possible that it could create some swelling of the tissue and pain. I recommend that prior to irrigation, you must mix your solution by stirring or shaking the bottle and this will allow the solution to be more uniform.