Making sure I ask all the right questions

Mid-size mixed breed MacKenzie and I are going to the vet this afternoon..
Symptoms include:
Occasional vomiting (yellow/white goo looking) first thing of the morning
Refusing to eat meals
Straining to poop yesterday (seemed "normal" at first, but it was dark, a little hard to see... she walked a few steps, squatted again, walked a few more, squatted more, about three times before coming back inside)
Lethargic
Warm, dry nose

At first, I thought "food allergy", so I started mixing the current food with something new. Her appetite increased for a few days, but then went right back to normal.

She was vomiting hard Sunday morning, refused to eat breakfast, then laid around the house 90% of the day, sleeping hard.

What should I make sure the vet is looking for? Anything in particular?