You must have something installed or someone else might have...I have not seen a true start menu in Windows 8/8.1, I am highly skeptical that it just magically appeared for only you and no one can confirm this for them...this perks my interest purely because of how many Win 8/8.1 workstations I have used, worked on, installed, updated, configured, etc. Not a single one has a Windows 7 style start menu without another program being installed. I prefer classic start and do an install of that, runtimes, browsers, MBAM, etc. from Ninite.com.
Screenshots please, left click and right click on start button. Also verify the listings in your Programs and Features.