My 4 year old has a very ichy scalp. she doesnt hvae lice. nor does she have danriff. i use danriff shampoo an?

she doesnt have danriff or lice (ive checked) i have cleaned her hair and went through it. she keeps "digging" in her hair and wont stop. its so bad that she makes herself bleed sometimes! doctor says nothing is wrong but i dunno. she doesnt comnplain she just iches! what can i do. i have used danriff shampoo and tar shampoo but nothing seems to b helping