Ok, I'm not exactly sure whether what has happened is something that sometimes occurs in the game based on certain situations or another one of many glitches this awesome game unfortunately has. Basically, the maximum Wg I can carry is 300Wg (I think that's the Max including the 'Strong Back' perk). I was wandering through one of the many underground Metro stations when I was attacked by a Feral Ghoul Reaver (buggers to kill), and after I killed it I suddenly found myself moving slowly then being prompted with the 'overencumbered' warning. I couldn't understand why because I hadn't picked anything up. Then when I checked my inventry, I noticed that my maximum Wg had dropped from 300Wg to 270Wg for no apparent reason. I have full health and none of my limbs are crippled so I don't know why it has spontaneously done this. Why has this happened and what can I do to correct it back to 300Wg? Surprisingly, most of the glitches (if this is one) I've encountered, have only been since installing the 'Broken Steel' DLC...
If you're using power armour, it's possible it got trashed in the fight. Which means it won't be adding to your max weight capability until it's repaired.
Thanks bro, you were right, my Power Armour was trashed. Though repairing it only increased my Wg up to 290. I then realized the Feral Ghoul Reaver had given me a massive level of radiation... curing this sorted the rest out. Thanks again.