I am confused after getting an answer wrong in a test and then reviewing the subsequent lesson.   

In a recent test, this sentence was given to complete.   "No me gusto nada hacer ejercicio _____________."  My answer was "después de comer," when in fact the answer was "después de comiendo."   

I went back to review the lesson and the lesson states that after the preposition "de" comes the infinitive form of the verb to say the "ing" form of the verb.    Now I am confused as to why my answer of "después de comer" is not correct.   When do you use the infinitive?  and when do you use the present participle (i.e. ending "ando" or "iendo") after the preposition de?