Hey I'm glad to see I'm not the only one who had this glitch, must mean that it actually is reproducable, so maybe it will get fixed. On the other hand, I offer my condolances, because it is annoying as hell. Go check the Dawnguard bug list, it is on there I know, I reported it. Personally, I had to cure my vampirism about 9 times before I just got fed up and restarted. It is annoying, but the way my random transformations were spaced out, I was able to handle it at first. I do not know of anyway to fix it currently. Sorry to hear you suffer from it as well.
Out of curiousity, did you transform between a werewolf and vampire lord? From your post, it would appear you never did. I was a werewolf at first, decided to become a Vampire Lord for a certain part of the main quest, then got rid of the vampirism to beat Dawnguard and got my werewolf back. I ran into ANOTHER glitch that kept me from re-entering the Soul Cairn, so I became a VL again and was able to re-enter (others who have had this issue just can't re-enter period, just a heads up). I left and got cleaned again, and then after finding out I couldn't be returned into a werewolf again, I moved on. At that point, I started having this glitch, so I wonder if that was maybe the cause.
Also, this post would be better off in the Dawnguard Xbox support forum, just as a heads up. GL with whatever you do!