Laurie - we had a weight gain blip at around 6 mo, it was the time I switched to double sided feeding, but dd wasn't yet on much in the way of solids. I'm afraid I know very little on the allergy side of things, so can't comment there, but if the loss isn't dramatic remember the happy healthy baby signs - 6 wet diapers, skin tone, demeanour etc. Has she started moving more? It is of course something to watch out for, but if otherwise Ellie seems perfectly happy don't let the Dr freak you out.
Keeping her on the breast as long as she wants will only do good things for your supply. If she's drinking I'd really keep her there, if she's just enjoying the coziness, well, the choice is yours - how much can you stand?