Maintenance After Long Term Restrictive/Bingeing

Hiya! I've had a look through some old threads for advice, but feel like actually asking may be better, hope you don't mind

I'm trying to maintain after going through 5 years of Anorexia, and in this past year Binge Eating mixed with a Restrictive/Purging cycle. I've left the Eating Disorders team and I'm still working on everything else with a private therapist, at the moment my B/P is in control (haven't done it for three weeks!) but I've still been logging an average of 700-900 calories per day, most days of the week- including work (I work in retail). I don''t exercise apart from this, yet, because mentally I'm not in the right place without becoming obsessive. My calories mainly consist of vegetables, oatmeal and one cup of soy milk a day (every thing is counted). So the past few weeks my weight began to drop a little bit, since stopping the bingeing, which was going to happen; but this week I haven't particuarly increased much apart from a physically large lunch yesterday of Jacket Potato and Pinto Been Chilli; but I also bake. I haven't binged after baking which is great, but obviously I enjoy liking the bowl and having a bit of cookie dough (you've got to taste it, right?!).

So today I weighed myself (it's become a weekly habit which I'd like to stop) and my weight has increased by 1lb, which logically doesn't make a lot of sense after a week of consuming less than 1,000 calories and working- the only thing I can put it on is the lunch and bit of cookie dough. However, where has this come from? I Know I need to increase my calorific intake, because it's began affecting my blood pressure, so I've made an agreement with my therapist to for now achieve 1,000 calories per day- then in time increase, which mentally will do me good. But, I'm a bit concerned. Surely if just after a week of one large meal and baking I've gained 1lb, how much am I going to gain over the course of eating an appropriate amount of calories?!

During my initial recovery process, I gained up to the minimum weight for my height at 47kg up from 38kg (I reached this earlier this year) and I've had trouble even maintaining that- however, that has included binge/restriction processes. I now weigh 49.4kg and my BMI is around 18.6 (dead on the marker!?) and would like to maintain this weight if I can.

Sorry for the long ramble! Basically, is this my body gripping on to every thing I'm giving it- this increasing in weight, is it the large lunch yesterday, or is it the occasional cookie dough; and how do I increase my intake comfortably without putting on much weight? Is this going to keep going up, or be a long term thing, or will I even out?

Hope someone can help, I'm really keen to stay safe but I'm feeling a bit lost! Thank you