Is your LLC a single-member LLC without an election to be treated as a corporation for federal income tax purposes?
If so, it is a disregarded entity and you should likely have been reporting on Schedule C (Form 1040) on a calendar year basis.
Please provide more details, including how you reported and why you chose a fiscal year (and one that was a short year ending after December 31st).
If you’re a C corporation, the IRS lets you choose between the standard calendar year or your own fiscal year schedule.
If you’re a partnership or an S corporation, you need to get permission from the IRS before adopting a fiscal year.
you needed to file form 8716
https://www.irs.gov/pub/irs-pdf/f8716.pdf
a single member LLC is a disregarded entity so would be treated as a sole proprietorship that must use a calendar year.