Thanks for posting your query.
From your description there is a wide range of possibilities for your girlfriend's symptoms. The symptoms could be as a result of long term secondary effect of mirena
which could present as the above symptoms. One of the side effects of birth control is ectopic pregnancy
(the possibility is there as indicated by the manufacturers). To rule that out, you will have to perform first a pregnancy test then do a pelvic ultrasound
. But to me, given the manifestations of symptoms and given the fact that symptoms started after sexual intercourse, the most probable cause is acute pelvic inflammatory diseases (acute PID) which results due to infection probably of sexual origin. You need to get your girlfriend tested for STDs, E. coli infection etc. A pelvic ultrasound will be of great importance.
If the infection then appropriate course of antibiotics will remedy the situation, if ectopic pregnancy then surgery or medical management will be indicated.
Hope this helps