Income Tax Payment received after quitting employer

Dear Experts,
I am Indian citizen relocated from India to Belgium by my employer on yearly renewal contract.
The contract was Local Belgium agreement. The agreement was my employer pays me a Net monthly salary and any other Income Tax related settlements will be done by my employer. The employer was paying for my social security as well.

I quit my employer on December 31st 2018 as my last day and now joined on full-time opportunity with a local company.
Now I have received a document from Service Public Finances department asking to pay a huge amount on my income tax settlement. The document says personal income tax and additional taxes for my income for 2017(2018 tax year).
I am afraid, this came UN-noticed and now I am worried who needs to handle this? I am also afraid if i contact my ex employer  they might play safe asking me to do.
Can you please advise? I am not even aware of any legal expert whom I can consult on this.
Any help would be great.

Regards
Zabi

You should have salary slips from your ex-employer, don't you?
He also should give you a `FICHE Nr. 281.10` at the beginning of the year for the previous year, where you can see you gross/net what kind of taxes he paid.

I do have year end statements. However, my point is if the ex employer was taking care of my IT returns, now that I have received a document mentioning I have to pay back a amount, should this not be taken care by my ex employer itself or should I pay it?
I am wondering why did they send it to me and not my employer.

Your employer's payroll department is just a mediator. Taxation settlement is directly between you and the income tax department. Your employer deducts taxes from your salary based on certain assumptions and conditions. It is the income tax department who does the end-of-the year assessment and informs you (the employee) if there is a refund or if there is more taxes to be paid. You may work with your employer to see if the calculations were correct, etc.