I seriously doubt that you need to lose any fat but in answer to your questrion:
It is quite hard to tell if weight loss is muscle or fat. There are expensive fancy shemansy weighing scales that will give you a fat percentage but these aren't super reliable from the research I've done I think. They are, though, good enough to let you know the direction you are going in if you make sure to weigh under similar conditions each day (first thing out of bed is easiest). Fat Calipers are preferred by most people who have someone that they don't mind taking the rather intimimate measurements required for them
Obviously if strength is decreasing that can suggest muscle loss.
Anyway i would agree with the other posters insofar as you should first just try to eat more.
Nuts butters are great but I prefer the nuts themselves as they are great to just grab with some cut veg on the go. They also go great with chocolate
if you PM me an email address I'll send you a really great article I have from a website I have a subscription to on the subject of gaining weight.
A couple of tips from it
1) to eat more try distracted eating. So eat while doing something else like watching tv.
2) add sauces and oils to what you are already eating. These contain a surprising number of calories. Olive oil is good. Also whatever sauces you like.
I would not stress at ALL about fat until you get your bodyweight to stabilize
Best of luck and keep us updated
JS - They think it will open the door to folk like LordMuppet campaigning for a threeway?