Littlelamb, something similar happened to my ds2. Ds1 ate for england, bm, weaning and even now. Ds2 was good with bm but is just more picky about food so when I weaned him, still bfing, he lost weight and for some time seemed to stop growing.
my (lovely) hv asked me to go and see paediatrician.
Paed saw a baby who was happy, healthy and full of energy and interest in the world and told me I had nothing to worry about. She also said that if he didn't put on weight/height for a longer, sustained period, that she would want to see him again, but thought this entirely unlikely. We never did go back. He is still a bit skinny but pretty average height wise, not as much energy as (or more lazy than?)ds1 but I don't worry about him.
You absolutely mustn't worry about breastfeeding - I am still bf ds3 and he's 2yr 4mth ( I was never such an earth mother with ds1 or 2!). And if you've only been weaning him for a couple of months, I'm not at all surprised he wouldn't eat as much as HV suggests.
If night feeds work for you and your ds, then I should ignore hv. I did controlled crying with ds1 to get him to sleep through, and it worked so quickly he obviously wasn't hungry at night. Couldn't get cc to work with ds3, and he fed quite a bit at night until he was nearly a year.
Is your HV using growth charts for formual fed babies, that can be very misleading. I think something about the way that bf babies feed more regularly and use the fat in breastmilk makes them less inclined to lay down fat - and therefore slimmer than ff babies. That may be a bit confused, sorry, but I'm sure that bf babies carry on being slimmer long after they've stopped getting most of their nutrition from bm.
Wish you all the best, you've done a great job bf til 8mths, keep going as long as it works for you and your ds.