M.M.
Sounds like pressure on your vertebrae to me. I tend to have it through most of my prengnancy (and even when not pregnant). A chiropractor can help alleviate the pressure on the nerves in your spine, or you could try a massage therapist that does trigger point therapy.
Your erector muscles are working very hard, and sometimes they can become SO tense that they actually cause the bone (your vertebrae, in this case) to clamp down on the nerve. I good adjustment and massage would release that pressure. I'm especially thinking that it's a nerve because of your nausea. The nerves in your spine tie into your stomach and surrounding organs. If something is out of whack in your back, it can definintely cause nausea.
I've been dealing with this issue both during pregnancies and regularly for years, so I've had a fair amount of exposure to how the body effects itself and how you can help yourself deal with it.