This happens all the time to so many relationship. You have to show her that you will be there for her whenever she needs you and that she can trust you.
I had the same problem with a long distance relationship I had in the past. She lived in London and I was in the States. She mentioned it quite often that it's hard for her to trust me while there is an ocean in between. I tried to show her how much I care for her. Finding some
london florists and sending her a surprise bouquet, writing an old school love letter, calling her as often as possible. The little things count in this matter and show that you care.
My advice would be that you sit down with her and explain her that she can trust you and there's no need to be jealous.