Is her poo (ahem) light brown to yellowy and about the consistency of mustard or peanut butter? Or dark and really watery?
Breastfed babies have an amazing range of poo, it'll probably come out green at some point! But it is very runny and often seems a little explosive...When Isabelle had an upset tummy her poo was a lot darker and more like water (sorry tmi!)
As far as being sick is concerned, I would only worry if nothing seems to stay down. She may just be very sicky.
I would call your health visitor to reassure you, plus I'm no expert, but remember breastfed babies are actually less likely to get tummy bugs because components of your breastmilk protect against it.