X-ray Tetra - Sudden-onset "growth" Between Spine And Dorsal Fin

I've just noticed a new "feature" on one of my x-ray tetras - it's a bone/cartilage-like "growth" inside, immediately above and in line with the spine, underneath the dorsal fin. It looks slightly pink in colour and is about 0.5cm. It wasn't there yesterday and I examine my fish closely daily. If the fish weren't transparent, then I would never know.  It seems to be the same fish that, when injured externally, developed a similar growth during the healing process before it disappeared - this was further back in the caudal peduncle area.  This time, there is / has been no external sign of an injury.

No other symptoms so far.

Presumably no need for any concern unless anything else odd or odd behaviour occurs but has anyone any idea what this might be?

Thanks, Sue; this now seems to have resolved itself ie the "growth" seems to have vanished over the course of the last week or two.

Thanks, Alex - yes, a bruise (from some duelling with the other smallest tetra which occurs from time to time) definitely seems the most likely explanation.