Saying I love you?

So I have this friend that I've known for a long time, and realized a few months ago that I really love her, but I always knew she was straight so I am content not telling her and just sticking around in her life and help make her happy when I can... Anyhow though we were at a gas station one day and she did something cute and I (just joking around) was like awww I love you, not thinking of any consequences. Ever since then she says it everytime she sees me. Everytime she says it I feel my stomach in my throat... I can say it back but it's almost forced, seriously that word scares the living hell outta me, even though I know she means it in a "friend" way. I just dunno what to do?