I'm really confused because I don't see any details about the issue now other than your daughter having tummy pain at night, a skin issue being treated with cortisone, and sleep disturbance or sleep-wake cycle disorder. Plus other posters referring to "scratching until she bleeds" and apparently there was question as to whether she could be ADHD? Was the original post edited out completely and then rewritten instead of added to? I don't even see a thread history other than this one single thread.
I has the confuzzles.
What made you think she might have ADHD? Sleep disturbances and physical ailments such as the skin disorder and stomach pain wouldn't indicate ADHD. Behaviors that include too short and/or too long of an attention span, hyperactivity, in/ability to follow directions, in/ability to focus, if she's oppositionally defiant, etc.
Skin issues, especially if she has a rash, could be due to allergies to something. Does it seem like it could be eczema? Perhaps if she's having this sort of skin irritation coupled with the upset stomach and pains, especially if she has diarrhea and/or constipation, she could have a Milk Allergy where she's allergic to milk or she's allergic to Milk Protein/casein. Or she could have the allergy AND be Lactose Intolerant (can't digest milk sugar aka lactose).
Other causes for skin and stomach upset besides stress responses: sugar substitutes like Splenda, Truvia, saccharine, aspartame; too much sugar in the diet; wheat and/or gluten intolerance; sensitivities to chemical food colorings and high fructose corn syrup even if she never seemed to have reactions to these things before. Even insect stings or seasonal contact allergies. I've had stomach cramps from having a stomach full of postnasal drip due to my allergies before.
The thing about allergies is that with each new exposure, they get worse than the last one, so they can start off very small and not noticeable or easy to explain away. But then they start getting bigger and more serious.
If she's stressed out or anxious, that can cause eczema and skin reactions. It can cause physical pains especially in the stomach. It can certainly cause sleep disturbances.
Melatonin is safe to take, although since she's a child I would start with 1/2 a mg and if that doesn't work, the next night go to 1 mg, then the next night up another 1/2 mg but I wouldn't go more than a 3 mg dose. You can buy it in liquid or you can buy it in tablets that are in 1 mg or 3 mg (that I've seen at CVS).
Our bodies naturally make melatonin: it's the hormone that our brain makes that makes us feel drowsy and sleepy and then makes us yawn to let us know "Hey, doofus, go to bed." Some people, like insomniacs and people with Autism, don't make enough melatonin so they take the supplement. The supplement is non-addictive and it clears the system within a few hours of taking it. It's safe for children and adults. I have to give it to my middle daughter, who is 8 yrs old and has Autism. I trust it implicitly but I also take it with her pediatrician being aware of it as well as her neurologist.