I think it's because you did hardcore mode and you need water. Or food,of course No I checked, I'm in regular mode so that's not it. If you are playing Dead Money, it's the Cloud. It will eat your hp over time (unless you're inside a closed compartment). You must eat, or drink, or use stimpaks to counter this.
You're carrying too much weight
No it doesnt matter where I am. I could be in the 38 and it would still fall, my weight is 200 under threshold.