
I'd also go with the possibility of a Pyo which can happen soon after a season. With the open sort, it will be obvious from the nasty foul smelling discharge, but sometimes the Pyo can be a 'closed one' which has no visible discharge going on ( much as the stomach can be rather swollen). The bitch will be lethargic, perhaps drink more and generally unwell. I'd not rule this out.
Even if she's not actively nesting, she could be going through a false pregnancy so I'd not rule that out either, and if this is going on, chances are she may do this after every season she's allowed to have. So get her spayed as soon as you can - and in case this is a Pyometra, please have your vet examine her asap.
ps The loss of another pet in the household can affect those left behind, depending on how close they were.