Neurochic, you are right in thinking that citrus fruits are acidic because they contain citric acid.

However, your body contains about 30-40 litres of fluids which, as I mentioned above, are kept close to pH 7.4 through well-understood physiology.

A few mL of lemon juice (about 0.1% of your body fluids) will have a negligible effect on your body pH. The same quantitative reasoning applies to the allegedly "acidic" foods which most people eat in moderation, with no adverse health outcomes.