Why are my teeth hurting whenever I remove Invisalign?

I'm on tray 5 for my Invisalign. I've been wearing this set for almost a week. A couple of days ago a couple of my front upper teeth hurt really bad whenever I remove or insert my Invisalign tray. They never hurt before. Why are they hurting now? I've been removing and insert my tray the same way as I always have. The pain lasts for about 20-30 minutes.

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