Dandruff? Shampoo??

So this happens when I skip a shower for a day or more. My head gets itchy and my hair gets very greasy. When I scratch my head I get either dandruff(dead skin) or shampoo from another shower under my nails. If I shake my hair around it drops and this is what it looks like. I wash my hair very thoroughly so I don’t think it’s shampoo. But it couldn’t be dandruff because on normal days there’s only like 2 tiny pieces of what’s in this picture in my hair. Help?