Why do my moods change so strangely?

This morning I was feeling really down, really paranoid, really jealous. Felt like the world was crashing down on me.

My boyfriend comes home from lunch, puts his arms round me and says he’ll miss me this afternoon and all of a sudden I’m ready for anything, could take on the world.

But I know, that just like that, I could get a thought in my head and I’m back to being down again.