Final answer:
The classical signs mentioned - one-sided lower abdominal pain, diffuse abdominal pain, and referred right shoulder pain - point towards ectopic pregnancy. This situation occurs when an embryo implants itself outside of the uterus, typically in the uterine tube.
Step-by-step explanation:
The symptoms in the question - one-sided lower abdomen pain, diffuse abdominal pain, and referred right shoulder pain - are typical of an ectopic pregnancy. In the vast majority of ectopic pregnancies, the embryo doesn't complete its journey to the uterus and implants itself in the uterine tube, a condition referred to as tubal pregnancy. However, there can also be ovarian ectopic pregnancies (in which the egg never leaves the ovary) and abdominal ectopic pregnancies (in which the egg was 'lost' in the abdominal cavity during the transfer from ovary to uterine tube, or when an embryo from a tubal pregnancy re-implants in the abdomen).
Placenta previa and gestational trophoblastic disease do not typically cause these symptoms and usually show other distinct signs. Appendicitis, UTIs, and PID are also possibilities, but they typically come with additional symptoms which aren't mentioned in the question.
Learn more about Ectopic Pregnancy