Sorry your dd is going through this, Lisa.
My dd is 10, with AS, and goes through periods like this. There was a particularly bad episode when she was also 8 -she's so active, but just lay around, went off her food, didn't have any interest in going out.
We got in touch with CAMHS and. as other posters have said, we were told that it's definitely not unusual in children with ASD, particularly girls. They were pushing to medicate her (prozac) but I resisted because I wanted to try other things first.
I don't know if this will help, as all of our dc are different, but I did the following, and she came through it after a few weeks :
. Made sure she was getting plenty of sleep at night -usually she needed a really warm bath and dh or me to lie with her 'til she fell asleep.
. Diet -small, regular meals/snacks as she couldn't face big meals. She would have a few strawberries and grapes in a bowl, then some dried cheerios a bit later, or a smoothy;etc. This was hard, because the food she actually fancied eating was junk food, so we had to steer her away from it.
.Exercise -we sometimes had to drag her to the park or for a short walk, but it definitely helped her to relax a bit and lowered her anxiety.
.Limiting stress by reducing any activities/visits etc -again tricky, because we didn't want to isolate her, so we cut back to a large degree but still encouraged her to do things like meeting friends for 20 mins at the park. However, we postponed things that caused high anxiety eg she was due to visit a new school.
.DD has loads of sensory issues, so we tried to alleviate her feelings of overload by keeping things as calm and quiet as possible eg turning the TV off, keeping her room tidy and tranquil etc. As she improved, we gradually reintroduced more stimulation, but she will never cope with 'normal' amounts anyway.
. Reminding her that it's ok and normal to feel down at times. She didn't know why she felt so rotten, so we just reassured her that it would pass in time and gave her extra cuddles.
I realise this all sounds a bit like teaching your Granny to suck eggs (!) and we all do a lot of theses things as a matter of course anyway, but I found we had to step it all up a gear when dd was depressed.
I really feel for you and your dd, it's so heartbreaking. DD is also really struggling at the moment and it's horrible.
I hope your dd feels better soon. Of course, if you are worried, contact your GP or Camhs. Sometimes it just helps to have someone to offer advice or a fresh pair of eyes xx