My load limit has dropped all of a sudden while I have been fighting raiders. I tried healing my limbs (which were not crippled). Is there something in the game I don't know about or is this a bug? I've tried searching online and couldn't find the answer.

your max load can be increased temporarily, off the top of my head, by:

-increasing your strength through chems
-wearing power armor

-wearing items that increase carry weight (pocketted, deep pockets, etc)

-if you have the 'lone wanderer' perk and you gain a companion, it should drop by 50

Ok I think I remember accidentally taking a Chem that increased my back strength. The effect probably just wore off. Thanks!