Do you have FB installed on your phone, which has her number saved in the contacts ?
Sorry, missed that part. Yes it is installed.
Sinz has hot the nail on the head...
If you have Facebook installed on a phone (even if punting phone), moree than likely you have automatically given FB permission to access your contact numbers or dialled numbers. If she has also linked her mobile number to her FB (which a lot of girls do, e.g. browsing FB whilst working), then the two of you will be liniked, and the other will be a suggested friend sooner or later.
Matters get worse if you have used your personal mobile or your work mobile, contacted the WG from it, and then she is linked to you.
AFAIK as well as mobile numbers, this also applies to email addresses that you have contacted from your phone or stored on the phone.
As Ballrag says, you get the option to hide that person or even block, so do that and move on.