I know my partner has days (or sometimes weeks) that are relatable to this. Sometimes it can be really confusing for me. But I've noticed that I usually find out she's been "sitting on" an emotion for a while; either avoiding the emotion itself or avoiding talking about it.
For example... .we had about 6 weeks of her getting upset over "little things", or even seeming to get upset over nothing at all. At the end of that 6 weeks, we had a public "blow up", and that's when I found out she'd been avoiding talking about something that had been bugging her for some time. All those "little things" were NOT the REAL issue at all; it's just that the REAL issue was sitting there silent and causing her to be severely irritable!
In DBT class, they are teaching her the difference between "justified" emotions and "unjustified" emotions. The "real issue" was very justified- she had every right to be upset about it, and it was something we needed to adjust in our relationship (and did, successfully). But all those "little things" were mostly "unjustified", which was why they confused me (and probably why they are confusing you, perhaps?)
BPD's have a very hard time communicating their needs effectively. In regards to the "real issue" in my example: she had been "trying" to communicate her need to me for 6 weeks, but she'd been ineffective at it. She'd been "dropping hints" instead of being blunt. She'd been saying, "It would be nice if _____ happened eventually/one-of-these-days... ." instead of saying, "I need this to happen by ______ date." It was like she needed someone's permission to be REAL about her needs, and we've discussed that at length since then. I've told her, "I will NEVER judge you for expressing a need, I promise. I might say "no", but I promise I'll do it with empathy and compassion." And since then, she's gotten much better at expressing needs in a "REAL" and concrete way.
I have found that MOST issues with my partner boil down to communication errors. Either one or BOTH of us was communicating ineffectively.
The only advice I have is try to pay closer attention when they DO try to communicate a need. Maybe ask some probing questions (if they are open to that) to try and clarify their need. You might find they are dropping hints you're simply not picking up. And that's NOT your "fault", or even their "fault". No one is to "blame", but there are things we can do as partners to help the communication become clearer.
But I wonder if any of that rings true for you? Could there be a bigger (and perhaps justified) emotion under the surface they are simply avoiding? Sometimes I ask myself the following questions, and maybe it will help some others too:
Has she had any recent nightmares or flashbacks that could have contributed to her irritability?
Is her job more stressful than normal right now?
Is there anyone in her life causing her abnormal stress right now? (Another family member or friend)
Did she eat today? (You'd be surprised how much skipping meals can affect emotions)
Is she ailing from anything physically? (Even a sprained ankle can make emotions more difficult to manage)
Have there been any major changes lately? (New job, new school, new ANYTHING can affect emotions)
Hope this helps
Good luck!
