Two top front teeth were moved down on invisalign, but while waiting for retainers, have moved back up in days after the attachment on one tooth was removed (even though i'm still strictly wearing the last tray). why would this occur/what should i do?

Teeth are held in bone by elastic fibers. Those fibers retain memory and it takes quite a while for the memory to abate (like a stretched rubber band). You'll need to consult with your treating dentist/orthodontist since additional treatment may be necessary.

The goals of Orthodontics are Function, esthetics, stability. Toachieve stability some movements, such as extrusion, require more complex retention. I hope you were being treated by a specialist Orthodontist, who is responsible for your post-correction retention. Speak to him/her. If treated by a General Dentist you may need specialist referral. If by a mall store, I. an only wish you good luck.