VentiLatte

So I'm new phD student as an RA receiving stipend. My offer letter says I get $11000 per semester with 5 installments (so I supposed $2200 each) but now my first paycheck is only $1200 and its period is 9/1-9/30... I started working on 8/16 till now its already 10 days.... Anybody got the same issue? I asked our department but haven't gotten any response yet. I'm really confused how much I'm actually getting paid.