What Causes Allergic Rhinitis With Headache?

Hello Doctor, this is Ragavi, I am 23 years old married woman. I am in US from around 3 months now. I have been having slight headache since last two weeks and nose blockage too on my left nostril from the last two to three weeks. On further checking the deep inside of my nostrils I found out that there is a pink swollen growth in the left nostril, which is almost blocking the airway. On searching about this in internet, I found out that this condition is called Allergic Rhinitis. I am worried now and want your help to know more details regarding this and how to cure it, prevent it and avoid it in the future. Also if you can let me know the reason as to why this kind of a condition occurs it will be helpful to me. I will be really grateful if you can kindly please let me know your views.