I filed a personal tax extension for 2018 but I also became an LLC in 2018. Do I need to file an extension for the LLC?

What kind of LLC?  Do you need to file a separate 1065 Partnership/Multi Member LLC or a 1120 S corp return?  Or is your LLC a Single Member LLC?  Did you select to be a S corp?

Yes, you need to file an extension for your LLC.  To file for an LLC extension, file Form 7004: Application for Automatic Extension of Time to File Certain Business Income Tax Information, and Other Returns