How tall are you darl? I agree that over 3L a day of water is excessive unless it's the middle of a heatwave. Also (not just because i avoid red meat) but mammal protien is harder to digest than fish/poultry so if you have constipation issues I would maybe consider not going too heavy on the red meat... it can really bind you up. I'd say some fish and some brown rice (avoid white/processed foods in favour of browns for better digestion too) would do your system some good, for example.

I am about 170cm and 54kilos would also put me in the "unhealthily low" catagory. I tend to hover around 65-66 and am happy with my weight on the whole... my bones are heavier than most women, you can tell by the size of my hands. What kind of frame do you have?