To feel so sad and overwhelmed

Got my BFP last Thursday morning and by my last AF date I'm 5 weeks pregnant. OH took a few days to get his head round it but is now over the moon and so am I. It was our first month TTC and we were both worried it may take a long time. We are of course so so happy we've been so lucky.

Problem is- I keep feeling waves of hopelessness and sadness. I keep feeling terrified that this won't last and overwhelmed with anxiety. I've been having dark discharge on and off for over a week but testing a strong positive still.

AIBU to feel so sad at a time that should be so happy? Or is it safer to not be happy yet and wait a bit longer?!