Whitening pens and blue light creating stains

[Marigolden]

Since using whitening pens and a blue light I've had stains appear on my teeth that weren't there before. So while my teeth are whiter now I have prominent dark spots, most noticeably on the teeth directly next to my front teeth. How did this happen? And what should I do to fix it

 

[Marigolden]

Here are 2 pictures. The left is before using the pens and light and the right is after.

 

[Dr M]

Good day
When last did you have a check-up with your dentist. Although this might only be staining that is more visible now since the rest of the teeth are now whiter, it might also be interproximal cavities. Difficullt to say for sure, since the quality of the photos are not clear enough, and you would need an x-ray to evaluate the presence and extent of cavities.