Vitamin b shot question.

I posted this but it has not shown up, please forgive me if it goes in twice. 

I took my cat to the vet today as she was not feeling well, I found out she has a cold, her fever is now gone, that is good, she is lethargic and has no appetite. She was given a Vit B shot, my question is. How long do the effects take to get my cat to feel up to eating again on her own, or will she need more than one shot? Or will kitty eat when she feels better to do so? 

Also she has not passed a poop and I want to add some pumpkin to her diet, I have been reading up on it and want to make sure I get the right kind. I will start with a very little at a time to kick start her. But want to know if this is the right kind in this photo

Yes, you want to get the kind of pumpkin that's just plain pumpkin, no spices added, like in the photo you posted.  I'd get organic if you can find it.  

I can't answer about the vitamin B.  Hopefully someone else can answer that question for you.  

Thank you so much for the answer, kitty did finally poop a little today so that is good. She is getting so much better each day, so hopefully the Vit B Shot is working.