It is really hard to pin point Why is it happens. I find myself going through the same emotions and in fact my fiance sees it alot too and just has now being making comments about it.

PlatinumHeart said it best, in these moments, as hard as it is, try to find something to lift you up. If you see a Therapist or Doctor, Talk to them about how you feel, and brainstorm ways to "cope" with these feelings Goodluck and know we are always here!!