Low weight gain

My almost 3 week old only gained 40grams in a week (1 oz).  The week before had gained 160 grams, after we had supplemented 1-2 feeds with formula as I have a low milk supply (according to LMC and lactation Consultant), in which I am taking medication and protein shake with a mixture of things to help supply. 

Since last Wednesday she has been totally breast fed and certainly not the hungry constantly feeding bub i had the week before.  So am wondering if I should be concerned.  LMC did not appear too worried, just said to feed her 3 hourly during day and 4 hourly at night (she had been sleeping for 5-6 hours at night so I thought was satisfied).

Are there plenty of wet/dirty nappies? If you weren't having baby weighed regularly would you be worried about baby's overall health?
I do remember that when DD was weighed at 12weeks so hadn't put on anything for 3 weeks. She had however gained in length and head size.
And I imagine your milk supply may still be building back up again after cutting out the FF.
If your LMC wasn't worried, I probably wouldn't be either. If however you are worried about babys overall health perhaps let them or your GP know.