I’m so sorry for your loss. I know exactly how you feel. My beloved dog Pecan passed away unexpectedly on March 20th. Just like you precious Simon she was lethargic and was drink lots of we’re but no other signs. It started at 6:30pm on Thursday and she passed away at 4am on Friday. I called a few places they said to keep an eye on her and it’s a good idea for her to be seen but it’s not life threatening. I made an appointment with her own vet for the next morning. I noticed her breathing changed around 2:30am and she fell when she wanted to follow me. So we rushed her to the emergency but she passed away before the vet even got a chance to check her. They said heart attack, stroke or aneurysm possibly cancer. I still don’t know what took her from me and I feel so guilty for not knowing she was sick. She was only 9 years old and was my true soulmate and best friend. I’m sending you love and light. Please take care of if yourself and allow yourself to grieve.