Hey RangerGuy..

My first suggestion is that you need to get this addressed with her & SOLVED prior to marriage..

Second, there could be a large realm of reasons. The first couple that come to mind:

1) Hormonal.
2) Depression
3) Stress
4) Problem within the relationship

My only real advice would be for you to talk with her about this. Not in a way that will make her feel at all guilty or obligated but in a way that shows her that you love her and that you are honestly concerned.

HUGS