I am over-encumbered

If you are under the effects of a burden spell, it should show up under the magic effects you are under and the spell should dissapate fairly fast. So I think it is much more likely that your stength level has been drained (bonewalkers tend to do this if I remember correctly). If you go and take a look at your stats, your strength level should be in red, and be lower then normal. There are a number of ways you can rectify this. Firstly, either travel to a imperial alter or dunmer temple altar and pay to get your stats restored. The other way is to simply create a restore attribute spell (strength in this case). You should be able to buy restore attribute spells from at least one of the many mages in the various mage guilds. Once you have one you can then create a range of spells to restore certain attributes. The last way is to make/buy restore attribute potions, I never do but I'm sure this way works fine.

This may be a glitch. But maybe you have uneqipped something that was boosting your carrying capacity, like a ring of feather. But the most common problem is you have picked up a disease from it - Black heart blight - ouch! Well the way to lift it would be to use cure common disease or blight potion. Or a spell that will do that. I really think you might be stuck without this sorry.

It must be a glitch. The bone walkers knacker your strength, and hence your carrying capacity, but after using restore potions/spells the amount I can carry doesn't recharge, which is irritating as I have kitted myself out with ebony armour and a daedric dai-katona... I LOOK impressive, but if you come any closer i'll bite your kneecaps off!