..you haven't discovered very many word walls to learn new shouts? I've killed 6 dragons and have 2 shouts and only have discovered 1 wall. The other shout was given to me by High H.

They just pile up.

Game menu -> Magic -> Shouts -> Lower corner shows your dragon souls

You hold onto them until you need them.

I was running on no sleep late last night and didn't notice.

Not sure if it's a bug or not, but I noticed with my game, if I don't have any unlocked words, I don't absorb the soul, infact the dead dragon does not even decompose, it just lays there looking all dragon like and stays that way. If I learn a word later I still don't absorb that soul, but my next dragon I kill will absorb the soul and I can pile up on souls as long as I have an unlocked word of power.

Had u been to the area where u killed the dragon before? Because if u kill a dragon that respawns on for example a previously cleared word wall, the new dragon won't give u its soul. So random dragons in the world u absorb all souls, on dragons fixed to one location (word wall) u only get a soul for the original dragon not the one that later respawns there..

*shrug* it's possable, been to a couple outdoor word walls where there were no dragons that I may have killed in it's flight path.