Sensitive parts of teeth at gum baseline, but x-ray didn't detect them - are they cavities?

[ZDundun]

I have a few very sensitive parts on some teeth that fall right along the base of the tooth where it meets the gum-line. That is, either on the very top of the tooth (top row) or bottom of the tooth (bottom row). Brushing over those parts of these teeth produces a sharp pain on contact (they feel like cavities). The remaining surface area of those teeth feel otherwise normal.

I recently had an x-ray done at a dentist visit, but it didn't pick up any of the mentioned teeth as cavities. I scheduled a second opinion at another dentist, and she suggested that the reason why they might be hurting is because I'm not wearing my retainer every night, which causes the teeth to shift and therefore cause pain in those areas. I wore braces for 2 years, and my habit since then was to wear a night-time retainer every other day. Since that last visit and her suggestion to not skip on wearing it, I've been wearing my retainer every night, but my teeth still hurt just the same.

Is it possible that for an x-ray to be wrong and not catch cavities? Based on my description, do the teeth I mentioned sound like they are cavities, or is it really true that if I continue to wear my retainer every night the sensitivity will eventually dissipate?