You should do a pregnancy
test with ultrasound
examination of pelvic organs if it is possible (to exclude pregnancy). It could be desirable to know which emergency contraceptive
your girlfriend took. There is probability that she has cyst of ovary (do not forget that now her single ovary has double pressure, it works for both). She should avoid much physical activity, sunlight, alcohol these days before the situation becomes more clear. If functional ovarian cyst
revealed it can disappear spontaneously without any treatment during next cycle. Other ovarian formations need individual strategies.
Wish you Good Health