understanding bpd illness

Is your gf on medication? That is key to her moods being more stable thus her responses to life being more stable. Many people with bpd have episodes of irritability and agitation. This can be the result of mania. The brain is flooded with constant stimulus and this causes irritation. Lack of sleep on top of this will only compound the mood. Proper management of the disorder is necessary.
Get some books on bpd. It will help you recognize the signs of mania and depression. There are many and they present a bit differently in everyone. Mania is hallmarked by pressured speech (constant chatter, won't shut up), increased energy, less sleep, hypersexuality, impulsive behavior (impulsive spending, taking a road trip out of the blue...), risky behavior, grandiosity, raging, and irritability. There are other symptoms but these are what come to mind. Depression presents with low energy, lack of motivation, hopelessness, helplessnes, increased sleeping, poor appetite, loss of interest in activities that ususally are enjoyed, and suicidal idealogy. Again, there are more but these are common presentations. Even with all the reading you may find there are times you still don't understand the disorder. Give her room, be supportive, and learn all you can. Support groups can be a source of information. They can offer you ideas for coping with your gf. Be sure to take care of yourself. This disorder can zap lots of emotional energy. You need to be prepared for this and have an outlet. BPD is challenging but can be managed. I hope this helps.