Gluteus maximus and quadriceps hurt suddenly for no reason. pais is lessened when standing, but getting almost unbearable on sitting or lying down.?

Check your hip: Hip problems typically cause thigh pain as well as buttock pain. However fractured hips generally hurt more when weight-bearing. Femoral nerve problems are often worse with hip flexion but shouldn't cause buttock pain. Any pain worse when recumbent is worth getting checked out, soon.

This is not likely : Be a hip or sacroiliac issue as the opposite would be occuring. This sounds most likely to be a high lumbar disc herniation at l3-4 or l2-3 or possibly a far lateral disc herniation at l4-5. Sitting is 4x your body weight across the lumbar discs & standing is only 2x. When lying down, the neural foramen may be more narrowed & the disc may extend into this location. You need to see a physician.