I have bad mood swings too. I may have a separate mood disorder, or it could just be a part of my BPD. One minute I'm ok, something or someone will set me off and boom! I'm in a massive mood. I'm in one right now. Because I've just rowed with my partner. I was ok before.
You aren't alone in this. I hate being moody, but other people have caused me to be like this. You can always PM me if you need.