*And when I'm back at work at the end of the year and have money again they will increase the tax credits based on this year?
When we don't need it as much?!
That doesn't make any sense at all*
It's the big flaw with tax credits. Because they're assessed annually, they're always based on estimate income. Unless HMRC have been told otherwise, they use last year's income to forecast this year's. When they send out the review paperwork, it's important to check that last year's figures are right and that this year's estmate is plausible.
The reviews have generally gone out by now, and now's the time to correct this year's underestimate, OP. When you get next year's review, if your income has gone, or will go, up, make sure you let them know, or you'll be overpaid.
Are you renting, and do you get housing benefit? If so, make sure you tell HB that your circumstances have changed, as your HB will go up.
You can't make a new HB claim now, but you may be entitled to help with your housing costs if you claim UC. UC doesn't have the problems that TCs have, because it's based on actual income. (It does, of course, have other problems!)
I'd run those figures through one of the online benefit calculators as you may be better off on UC.