i think it could be a combination of what richard said and a potential back injury...had to say for sure (at least for me) from just the video. notice that the pelvic fins are just flopping around...although these fins are not used as much as the pectorals in general movement, it looks like there are some control issues there...could be related to blockage and/or sprain/damage to the nerves in that area.
that being said, follow richard's suggested treatment since if there is other damage (i.e. sprain or break) it will heal on its own naturally if it's anything that can be recovered from--
--solomon