Braces off but my teeth are moving back, how to prevent this?

Call orthodontist !! Retainers should be keeping the teeth from moving. Call the orthodontist immediately for a follow up to prevent any further relapse.
After Ortho. After orthodontic treatment is complete, some type of retainers must be worn every day in order to keep the teeth in place ; prevent them from moving back as they have a "memory" ; want to do that. If you just completed treatment, your orthodontist should have provided retainers ; discussed retention. If you completed treatment awhile ago, you will need to return for treatment ; retainers.
Retainers. The most common way to maintain straight teeth after removal of braces is using retainers, but this requires diligence and tolerance. Another route is permanent braces placed on the lingual side of the teeth so they do not show on the outside, but this may be difficult to adjust to and clean. Consult with your orthodontist to see what best fits your condition and lifestyle.
Variables. Unstable teeth may be due to: 1 Treatment done by generalist rather than Orthodontic Specialist. 2 Failure of patient to fully cooperate. 3 Compromised oral hygiene with resultant damage to supporting tissues. 4 Failure of practitioner to provide you with retainers. 4 Failure of patient to wear retainers EXACTLY as directed. 5 Treatment with a less effective appliance, such as clear aligners.
Braces. Depending how long after the end of orthodontic treatment. Would go back to the treating doctor and consult on options. .