4 months no period and the doctor wont do her job what should we do?

My girlfriend hasnt had her period in 4 months. taken multiple pregnancy tests and the doctor insists shes not pregnant even though she believes its possible and is suprised the results are negative. she has all the symptoms and says she has that feeling of knowing shes pregnant. the doctors said she might olf just miscarried and not bled but still wont give her a sonogram. they keep trying to take percocet and birth control to see if they work but were scared shes pregnant and dont want to hurt the baby. if she took them or fails to get the baby proper attention and something gloes wrong what happens? is there anyway we can get an ultrasound? her mother took multiple pregnancy tests while she was pregnant and they all came out negative also. any info would be greatly appreciated. thhank you so much!