How often? Sex..

We are having some months were we have not had sex very often (probably once every 2 or 3 weeks). It started because I had a problem in my back which was very painful. Now I am ok, but we are just too busy or tired. Is this normal or should I worry about it? We love each other, we kiss and hug everyday. But we have only 2 years together, I don't want this to be worst in the future.