My fiancée has always had frequently occurring severe muscle knots that cause nausea and vomiting. What specialist would be the best to visit?

IM, PMR, Rheum. Things you can do first: maintain proper posture, use ergonomic workstation, yoga, exercise, apply heat pad to the "knots". If not better, then see your primary care doctor. He can diagnose and treat you or refer you to a specialist, either PMR (physical medicine & rehab) doctor or rheumatologist. You may also benefit from physical therapy.