There is no reason why "I am studying" is incorrect here, it does not mean "I am studying right now" as "estoy estudiando" would or "estoy estudiando ahora mismo" would but in context it works. "I am studying for my GCSE's this year" for example. I don't have a link unfortunately but Michel Thomas explains this very well. Please note that "I study" is also correct and does make sense in English contrary to some comments. eg "I study every day for two hours"