How can this be?

I have hurt my back and have been unable to exercise. I can barely walk! My calorie intake is the same - around 1200. I have gained THREE pounds in two days. Certainly my metabolism is not 0. I have not eaten enough calories to produce one pound in those two days.

Where is the weight coming from?

Well you simply cannot have put on 3lb of fat. Don't stress over it, just worry about getting our back sorted.

It could be poo, when did you last go? It's heavy stuff you know.

Also, injuries tend to develop swelling, and fluids are heavy.

Same advice as usual too - always weigh yourself at the same time, first thing every morning, naked, before having a drink (that coffee could weight 1lb, what's the point of weighing that?) and after going to the toilet (that pee could weight 1lb, what's the point of weighing that either?)

Are you on painkillers or other meds that could make you retain water or make you constipated? Painkillers usually do this to me?
Or it's your body having some wacky response to injury, and change of routine. don't sweat it, you know you haven't really gained 3 pounds!

Not being able to walk around you probably aren't shifting food through your gut at a normal rate. Walking helps your intestine to work, so it'll be water weight and food in there.

THIS.
Also I wouldn't advise weighing daily. Your weight fluctuates too much and it will only make you crazy. I weigh monthly but if you must weigh frequently then no more than once a week.

It's two days.

If you weigh yourself everyday (and I do) only take any real notice of what your weight is every 7 days. I can weigh 4lbs more on Thursday than I did on Monday, it's how my body operates and I don't know why.

Give yourself a break.