My morning sickness would come and go also, I know it wasn't every day. Having had two pregnancies and a bunch of miscarriages under my (now super-sized) belt, when I knew I was having a miscarriage was when all my symptoms disappeared at once, ie. sore breasts, constantly being tired, having to pee all the time etc. Like Kristen said, my morning sickness would come and go based on how much I had eaten or drank - worse when I let my stomach get empty.
Congrats on your two new little beans!