There is nothing "stronger" or "weaker" about Yasmin. While all birth control DOES affect hormones, there are lower dose hormone pills that are available, which is the origin of the name "Ortho Lo" as opposed to "Ortho Tri-Cyclen". I believe Yasmin to be on the "low dose" range, but that doesn't mean your girlfriend's hormones are untouched. She is taking a pill that consists of hormones, which obviously will result in change of hormones.
While I took "Ortho Lo" I was miserable, I then switched to "Alesse" and had troubles 3 out of 4 weeks. Your girlfriend probably doesn't realize her change as heavily as you do, and she probably doesn't know the full extent of her side effects from the pill. I have invested much time into this, now as I am off the pill, and while I was taking it. Even after reading about the negatives while taking oral contraception, I was not fully aware of the way I was acting, as I focused on how upset I'd feel. She needs to see a doctor and find something more suited to her body chemistry.
With that said, her birth control MAY be the problem in your relationship, but that's not a guarantee.