I used Philips night white. It burned my gums, they swelled 2.5 years ago. The swelling never went down. Why?

Have 16 crowns, 6 veneers; used Philips NiteWhite (22%) 1 time, 3 years ago (2014) (used several times previously w/ no prob,); trays removed next am, whitening leaked, swollen gums; 2.5 yrs, later, swelling never went down or changed to this day. My dentist used a laser to remove 100% bacteria, nothing; 2 periodontist visits and they couldn't figure it out. I have no pockets, my measurements are fine. Use waterpik, floss, electric toothbrush-- daily. Turned down for deep cleaning. Suggestions?