Thanks for your consult at DoctorSpring.com.
It is very rare ( unbelievable, almost ) that a pregnancy test would not read positive until the third trimester !!
All the symptoms that your fiancee is having, are suspicious of a pregnancy. She does not HAVE to take a pregnancy test if she is 5 months gone !
Please get an ultrasound scan done at the earliest. It would reveal a well grown fetus if she IS pregnant. Else, it would reveal the cause for her bumpiness and missed period.
Hope this helps, please feel free to discuss further.