Are you non-opiate tolerant? I take it you're just using this to get high and don't do opiates very often?
Well, headache and nausea are common side effects from any opiate, be it buprenorphine or a full agonist opiate like oxycodone or something, in non-tolerant people. Those side effects are especially pronounced if you're on too high of a dose, and if you are not tolerant to opiates.. 8mg's of bupe is definitely too much.