Kitty Lost Weight

I have a 3 year old kitty (Cinder) and she has lost some weight. She used to weigh 10 pounds. I'm not sure what she weighs now. I'm going to weigh her tomorrow. The end of July, I lost my 13 year old male cat. They were very close and got along like they grew up together. I had to put him down and it was a very hard decision. At the beginning of Sept. I adopted two kittens (same litter). Cinder was VERY mad, but now she is getting along a little better. They kind of play now and chase each other, sleep on the same bed during the day. I didn't know if this would be the cause of her weight loss with alll the change the last couple months. She's lost enough weight that it concerns me. She doesn't look unhealthy and she eats, drinks and sleeps fine. For the last month, every day she brings home a mouse (dead). I'm worming her tomorrow. Could all the changes in the last couple of months be the issue? I could use some advice. Thank you.

Is this a sudden weight loss or a gradual weight loss? It best to take her to the vet and see if there is a medical issue going on.

Chuckie lost a lot of weight really fast and we wound up loosing him from complications of diabetes which the vet was quite certain he could not have at his age (he was only 5 yrs old).

Please take her to the vets, weight loss is never good, and while it could be related to stress, I think she should be checked over just in case.