If any of my dogs were losing weight and there has not been a change in diet or activity, then you can bet that I would bring them to the vet, whether they were "acting weird" or not. Here is a sign -- you can see/feel ribs! That is a big red flag to me, and if my pets lost weight with no good reason (especially if you try feeding more and they are not putting weight back on) it is usually a sign of a more serious problem and is neglectful to NOT bring them to the vet in that case.