i would honestly (and i rarely suggest this) call back

check online services first so that you can check the estimate is correct, then check with them what CONTINUOUS ADJUSTMENT is in place and why

it sounds to me like the estimate at the start of the year may have been too low - how, i don't know - but now that it's being rehashed, it's saying you were overpaid for some reason. what you need to do is find out what estimate was on there July 1, what it changed to the few times you changed it, and WHY you are having money deducted.

if you feel you need to, to get someone to explain it in simple terms, play the pregnant card. say that you're feeling rather lost because you're sure you have over estimated in the past, and want to see how it's come to be that you're losing so much money. i have done it myself, played the "dumb" card. if the person doesn't explain it accurately enough (have pen and paper, make sure you write it down and it makes sense), PM me the info and i'll see what i can come up with - wouldn't normally take a PM in this circs, but i don't want you having to post personal info re estimate online....

make sure you also get from them confirmation of what your income estimate was last year, and what your actual taxable income was. hope the person you speak to is more helpful, if not, i'll see what i can work out