diarrohea?

Since Ebony had her kittens 5 weeks ago I've been keeping her in all the time, when she used to go out she hardly ever used the litter tray, presumably preferring someone's garden
Now when she uses the tray she seems to have diarrohea, I dont know if she had it before the kittens or not. She's always been a thin cat but seems thinner lately. She hasn 't lost her appetite at all, in fact seems to be hungry all the time. Should I be worried?

I'd definitely schedule a vet visit for her. She may have worms and if so she'll have also passed them on to her kittens.

If she's eating well and getting skinnier, then that's a pretty good sign something is wrong.

Cats are excellent at hiding pain so just because she "seems" OK, doesn't necessarily mean she is.