Getting too little pay after termination?

I was in an apprenticeship for three months, from which I was dismissed due to three unexcused absences (I was sick, but the medical certificate was not accepted).

I was sick in the second month, during which I was still receiving my full salary (about $900). In the third month, I was terminated after 21 days. So how is it possible that I was only paid about $300 for that month, even though I worked the same amount of time in those 21 days as I had in the previous months? That can't be right when I worked almost the full month.

Sick pay etc. which I did not receive due to unexcused absences cannot be explained if it is almost ~600$ that are missing