I put on in week 3 last time, this time I have only lost a lb this week so far, sometimes if your body has a big loss it seems to panic, but it will settle back down, you can't actually gain gain if you are only on 450-600 cals, you need more than that to survive.
On the toilet front - I use glycerine suppositories (they are totally not so bad as they seem and they work really fast) They are cheap from your pharmacy - just tell them they are for your dad if they ask if they are for you!! I usually pop one in (it goes in where it will come out!!) have a bath or a read for about 20 mins and then voila, and you feel so much better! You can also use senna - but I find that is not enough for me)
Also did you measure yourself to start with? I have lost inches, even if the scales aren't moving.
Hang in there, I agree with Nellyphant, don't go on your own scales (ok I don't follow my own advice) as it dictates your mood - there is a great thread on here about measuring success by the scales, it really helps in these low moments.
Hug - hope this helps x