I also feel like my body is made out of lead. I feel so heavy like a heavy log which I can't carry around. It is such a strange feeling. I also feel short of breath.
I relate to those (thou dont have the issues now, they've gone away on their own)... all part of the parcel of ME for me. That heaviness, I personally related it to do with something neurological as its far different to my normal tiredness/exhaustion. I find that one can have extreme exhaustion/tiredness happening yet not feel this other kind of body heaviness (rather then just tiredness) thing, so I think they are two completely different things.
eg I can be okay enough to get out of bed but be feeling so weighted eg say if its my head being too heavy on my neck, my neck then will be dangling either sideways or I'll have my chin sitting on my chest, due to the effort to hold it up due to heavy head.., (thou I may of been feeling okay and had gone out to visit someone ..back when I was still driving).
About a month back I did expereince this again but just with the heavy head, I ended up wearing a neck brace for the day to hold my head up, so it didnt affect the other things I wanted to do around the house that day.
Other times my feet feel weighed (like I had big bags of sand sitting on my feet or on the ends of my legs) to the point where I'll be dragging my feet when I walk and if I had to climb stairs.. I'd be picking one leg up at a time with my hands to place them on the stair as too heavy for legs to lift themselves.
Other times it was my whole body feeling very very heavy (at those times I just stayed in bed..too much effort to be moving at all.. even too much effort and heaviness then to roll over in bed.)... This heaviness thing can affect just one part of the body (at times when Ive got it in the arms it felt like my arms were going to fall off due to their weight) or the whole body.
I do get a lot of dizziness but only once have had very bad vertigo which came on suddenly and extreme but it went away just as fast. Ive no idea what triggered it.
I hope that B12 helps