I Got my Braces off a Day Ago and I Didn't Get a Retainer Should I Be Worried About Shifting?

I would not be overly concerned that your teeth are going to shift from Wednesday to the following Monday.  It is a very short span of time! Make sure you get your retainers this Monday and wear them on a regular basis.  It does not make any difference if you do invisalign or braces. After any ortho work you will need to wear some form of retainer at night for the rest of your life so your teeth do no shift back.  Just follow through and pick up the retainer!  Happy Straight Teeth!