The gap between my teeth is re-opening after 10 years of wearing my retainer almost every night. Flossing makes it worse. What causes this/what do I do?

Retainer. Your retainer may require an adjustment. Please see your orthodontist.
Things change. As an example you have a whole new skeleton every 7-8 years. Retainers significantly minimize change when worn exactly as directed. Space may be from changes in your occlusion, changes in the retainer, changes in your bone support, gum issues, parafunctional habits, etc. See the Dentist or Orthodontic Specialist that made the retainer for a reevaluation.