What's happening

I'm 7w1d.. Yesterday my husband and I went for the scan and saw a healthy heartbeat and we were very excited. The thing is, lately I've just felt so down.. For no reason. My colleagues at work think something bad has happened and keep asking, "what's wrong?" 
I have nothing to be sad about and I'm generally I'm a very bubbly person but I'm just down. I feel like i should be ecstatic about this time of my life. Anyone else experience this and does it go away?