I too am not sure what you mean by toothpaste brown. Brown is the normal color for stool and the consistency of toothpaste is normal as well. If she is eating less, having fewer stools is the normal result. Unless you can provide some additional info, I don't read anything that indicates a potty problem.
The lack of appetite and weight loss could well be from repeated heat cycles. First and foremost have her spayed and then if she is still losing weight, there could be some other underlying medical reason. I am assuming you haven't changed what you are feeding her. .
why on earth are you allowing kitty to be continually in and out of heat??? please do this poor kitty a favor and have her spayed, this is very hard on her and no doubt the reason she has lost some weight. please help control the birthrate and look after her health and have her fixed asap.
As far as her stool problems, do you mean the consistency of toothpaste? and how small? if shes eating less she will also have less stool, but the consistency should be firmer. what is she on for a diet?
It doesn't sound like a hairball problem, and since her stools are already on the loose side I wouldn't be giving any butter or oils,(do you mean mineral oil?) that will only cause looser stools and won't help with passing any hairballs if they should exist.
If you want to give her something occasionally to pass a hairball get the 'hairball' treatment in your pet shop, it comes in a tube like toothpaste and is only a few dollars...this is basically WHITE petroleum jelly with a malt flavor.
good luck, please reply so we can try to help you out some more with your questions...♥