Several months of very low food intake can genuinely cause this. Poor nutrition often leads to low protein, iron,
calcium or
vitamin D, which shows up as tired, aching legs and back, especially with exertion like climbing stairs. Your diet is a likely contributor, but new pain that worsens with activity is still worth checking isn't from something separate on top of it.
Next Steps
Start eating regular balanced meals with enough protein and
calcium as soon as you can, and get a basic blood test done, haemoglobin,
vitamin D and calcium, to see what's actually low.
Health Tips
Please see a general physician if the pain doesn't ease once your diet improves, or if you notice swelling, fever or weakness getting worse.