This sounds like you play on the PC, and I don't have much experience with that. I just thought I'd offer some thoughts for the heck of it. First off, are you using any mods? If you don't properly install your mods, you can cause any number of stupid issues. I would also advise you check your key bindings, just to be certain you didn't accidentally switch one of those around. Finally, I am uncertain how patching works for PC players, but it may be the case that you need to download an update to the game still. I'm sorry if all of those things sounded really obvious, I'm just trying to think of anything simple you might have overlooked. Hope it helps, more importantly, I hope you fix your problem so you can enjoy the game!
mine always lags, and if i switch while im walking, ill be holding my new weapon in the same way i held the first. (ex:holding a sword and shield like a battle axe)