Following left hip replacement surgery, is it common for piriformis and sciatica to hurt so bad one can't get out of bed, for 7-8 mths.?

Uncommon. If your sciatic pain has been going on for 8 months, you may be dealing with a herniated lumbar disc and an MRI of your lumbosacral spine would be worthwhile for evaluation and treatment decision. You could consult a neurologist or neurosurgeon first.