13/17 weeks is awfully early (well, by current guidelines, I know it used to be 4 months). They do plateau - my DS certainly has once or twice - especially if she has had a cold or anything (as every child seems to have had recently)!
I would be hesitant to introduce anything other than milk at this stage, and (contentious comment alert) especially just on the advice of an HV.
Personally I would hold off for a bit if you can; as you say, they do vary and I'd evaluate her on her skin colour (I mean, does she look healthy, not are you black or white etc!), is she lively, her sleeping patterns etc. If she looks and acts well, she probably is.
But if you have doubts, go and see your GP, that is what they are there for.
I am slightly anti HV at the moment, I know they do good work but mine told me catagorically that DS had "dry skin", and "definitely NOT excema", and he's just been diagnosed with, you guessed it, excema. So I would go and see a doctor by preference.