Mika - as you mentioned that you have been fertilizing your tree twice a month and with Miracle Grow too - I'm thinking that could well be the reason why your tree has lost its leaves - as apart from the fact that feeding your tree twice a month is way, way too much - Miracle Grow is not the correct fertilizer either - especially as all citrus tree varieties and especially lemon trees - even when grown in a pot - only need to be fertilized once a year - preferably around February to March and with a potassium rich fertilizer rather than one with a high nitrogen content.
I would also add that if you are also watering your tree as often as you are fertilizing it - that could also be contributing too - as even in the driest of conditions and especially during the winter months - lemon trees generally require no more than a through monthly watering.
Although I would agree with what Chuck has said regarding re-potting - mainly because a change of soil would help get rid of the residue from the Miracle Grow - it doesn't really matter what you choose in terms of potting soil - as lemon trees will grow in almost any type of soil providing its very free draining and not too rich