My lab is attacking her siblings

I have three dogs. A lab, chihuahua and a puggle. Recently my lab attacked my puggle and two weeks later she attacked my chihuahua. They have all been together for about a year and they get along great. They play, sleep, and eat together. I don't understand why she is attacking them. I see no triggers. The puggle was asleep on the couch when she attacked him and the chihuahua was just walking across the room. After she attacked, I locked her outside. When she comes in she lays down with them like it never happened. Why is she attacking them and how can I stop it?

I think getting your dog to the vet and possibly a behaviorist would be best. There could be some sort of pain thing or there could be a behavior issue. But without a professional evaluation I don't think any of us could even begin to guess what's going on.

I agree with Crysania, I would take your Lab to see a vet.
It rather sounds like there is an underlying issue here and you need to rule out anything medical.
Good luck and let us know how you get on.