My thumb nail on my right hand seems to be lifting from the nail bed and is a yellowish color on the top part of the nail. it is only on one nail.

Fungal infection: Sounds like a fungal infection, particularly if the nail appears thicked. Not dangerous, but hard to get rid of.

Many possibilities: Some causes of nail discoloration include trauma due to tight shoes, infection, and medication side effects. You should try wearing shoes with a higher wider toe box. Get the feet inspected by a professional to determine the cause of the problem, as treatment varies depending on the diagnosis.