How much weight has been lost? Has your baby perhaps become more active and possibly grown taller at the same time?
Babies often start to loose a little bit of weight once they become more active, if they have started crawling or walking then eating is not as much interest so they might eat a little less initially, and do't forget it is summer and very hot so eating is not as pleasant - perhaps try yoghurt and puree fruit ice-lollies as a way of getting food in that is nourishing, fun and cooling
If it is reflux, I think you would have seen signs of it earlier as reflux often gets better once baby is sitting, a little bit more mobile and starting solids.
Typically a reflux baby can be irritable after a milk feed, often cries when lying down, may vomit either in little bits or projectile, has a sour smell from their mouth, arches back when crying, refuses to eat or drink, prefers to 'snack' on small amounts of food / milk more often throughout the day etc.
HUGS