I hope she's ok!
If she has external sutures in place, then yes, use a collar. Most cats don't need pain medication after routine spey surgery, though she might well get sent home with some since her surgery was a pyometra. I would imgine she'll also stay on antibiotics for a while. Now that she is speyed she will be feeling so much better, so should be eating & going to the toilet normally when she gets home. Check her wound daily for any signs of swelling, oozing, or any other changes. But do be careful when lifting/handling her, as her abdomen will be a little tender for a few days post op. If the infection is cleared effectively, she should be back to her normal self after a week or so. It is advisable to keep your house warm, and have somewhere comfy for her to relax & sleep. Don't expect her to able to jump or climb stairs initially, so have her dishes/litter tray in easy access for her. Oh, and the most important thing of all.....lots and lots of TLC, and give her an extra snuggle from me!