Brown spots in my toe nails that look like bruises. what could be the cause? is it diabetic foot?

Trauma: To nails from shoes is most likely cause. See podiatrist.

Many possibilities: It could be a fungal or bacterial infection. It could be bruising due to tight shoes or trauma. It could be a pigmentation of the underlying skin. It could be a skin disorder or melanoma, or it could be one of a number of many other possibilities. If you are diabetic you have "diabetic feet". Have your feet checked out by a professional.