kittenbaby, don't wait to call the helplines, you need their backup sooner rather than later in the face of this appalling health visitor. I wasn't able to reestablish breastfeeding after early hospitalisation, but my baby was severely dehydrated as well as jaundiced, it wasn't just his weight.
You've got support of the "loony left wing brigade" on mumsnet (!) (ok, I'm definitely one and sadly I'm not breastfeeding) - does she also consider the Department of Health to be part of such brigade? - they advise that bfing until 6 months is a good thing. Try and get some rl support.
Ask if there is anything else apart from weight, which shouldn't be the only thing they look at - if your baby otherwise seems healthy and responsive, and clearly she is gaining weight now, then commonsense would surely suggest she's just fine.
I agree with everyone who's suggested you should complain, this woman shouldn't be in her job if she can't support women making different choices in bringing up a healthy baby. It's not her role to force formula on you just because it would have been her preference. Really, I'm shocked by this.