If she recently switched birth control then you're probably dead on.
For men who don't know: trying to find a birth control that works well with your body chemistry is the epitome of hell.
When you start a new birth control you're required to stay on it for 3 months, because that's how long it takes your body to accept the new hormones and begin regulating itself properly. The problem here (and speaking from experience) can be that if said BC isn't interacting well with your body then you're in for a looooong 3 months, and for nothing.
Spotting can be normal for entire 3 months. Every day. She may also experience breakthrough bleeding. This happens sporadically in between cycles. Its heavier than spotting and typically less consistent.
Retaining water weight is an absolute. While I have no science to back this up, typically I've found that a BC that works well with my body does not cause me to gain weight.
Finally, the most important aspect can be bouts of depression. A BC that is working well for an individual will help in mood/hormone regulation through out your cycle. However, if a particular BC doesn't interact well with one's body chemistry, then depression can be a very serious side effect that many times is over looked.
The best thing your gf can do is to be aware of these facts and aware of her mood in comparison to everything else going on in her life. If she is retaining some water weight (which could be perfectly normal until she balances out) and experiencing some mild symptoms of depression then no wonder she is feeling down on herself! Be there to be as supportive as you can be while she's insecure and make sure she does her research on not only BC in general, but the particular brand that she is using.
Also, make sure she keeps in touch with her gyno or physician if anything makes her too uncomfortable. Unfortunately, they will probably tell her to hold out to the 3 month mark, but if she decides to change at that point then the more info they have the more likely they'll be able to pinpoint a successful brand for attempt 2.