Just under the cuticle of my big toe is very tender, dark blue and pus like. what is the cause? toe feels unstable?

This sounds like an infection called paronychia, although the color (dark blue) is atypical. Are you sure there was no injury with bruising? If not, paronychia is likely and you probably need an antibiotic prescription and perhaps minor surgery to drain it. You'll need to see a doctor or clinic. See your primary care provider; or if s/he is not readily available, an urgent care clinic. Good luck.