That's the fun thing about O-care. People don't realize that if you don't "estimate" correctly, you have to pay the difference in a lump sum at tax time. It's bullshit.
My suggestion? I would try a different software package and run the numbers through it. I've found that different tax software will give you different numbers (happened again this year to me). I ended up working it by hand and went with the package that matched my figures.
It shouldn't matter what your take home pay is because Obamacare is based on Gross Income. You will however probably have to pay more in if you didn't change your withholding to adjust for more take home pay.
Regarding your bill, if all is correct and you do owe the $1800, the IRS will allow you to make payments (plus a little interest and maybe a fee). If you can't come up with it all at once, this may be an option for you.
Good Luck Anon - I hope this helps!!