Evening filthiestfis,
Has she been to her GP yet? Has she been poorly recently due to cold, flu, any bugs etc? You say about once a week.......for how long now?
If not the best thing to do,would be to arrange for her to be seen by a doctor ASAP and get checked out? it may be something quite simple that can be identified and will give you, and her, peace of mind too.
Best of luck and i hope she gets better soon.