Sick kitty with droopy lip :(

Sorry this might be a little long but I want to talk about everything.

I feed my cat Louis 2 cans of Fancy Feast a day, one in the morning and one in the evening. For when he gets hungry during the day or at nighttime I keep a bowl of Blue Buffalo healthy living out for him, although he doesn't really eat a lot of it. He's been on this diet for quite a while now. I ran out of the Blue Buffalo so last night I put out some different cat food that he used to eat (it's probably like 3 months old) until I could get some more today. He only ate a little bit of the dry food. Yesterday he was completely okay and ate like he normally does. In the morning when my alarm goes off he always purrs and meows and is really affectionate and when I get up he rubs against my legs and meows (I'm sure you all know what I'm talking about) but this morning he didn't even get out of the bed! He got up about 30 minutes later and walked into the kitchen and I got his can of food out and he didn't react at all (he always purrs and meows). I even bent down and opened it in front of him and he was not interested. He hasn't ate or drank anything all day and I'm worried  could this be because I changed his nighttime snack? Another thing I noticed when he got up was that his mouth looked a little off. I looked closer and noticed that one side of his lower lip was drooping. I went to get my boyfriend so he could see but when we looked his mouth looked normal again. Then a little bit later he was laying on my bed and his lip was drooping again  the side that is drooping looks like it might be a little bit swollen. What could cause this? He's been sleeping all day and hasn't gotten up in a few hours and I'm really worried about him  

I don't know what's going on with your kitty, it could have been the food.  Dry food can become contaminated with bacteria pretty quickly.  If he hasn't come out of it and started eating at least a little by tomorrow morning I would be taking him to a vet even if it means an emergency clinic.  I know how expensive they are but a kitty not eating for an extended period of time can be serious.  

Normally I would say to start syringe feeding but with the possibility of this being something food born I don't know if that's the way to go.

Thank you for replying! He's still sleeping in the same spot last night he was doing a lot of running around, do you think that he could have possibly ran into something that made his mouth swollen? Could that cause his lip to droop?

Whatever the cause, he needs to see a vet ASAP and you absolutely MUST get some food into him.  Try adding a bit of water to the food and making it the consistency of oatmeal and then warm it up for a few seconds in the microwave (about 5-7 seconds)  to see if that will help entice him to eat.  If that doesn't work, you're going to have to force feed him.  Water the food down a bit more and use a syringe (no needle).  Stick the syringe in between his gum line and his cheek and slowly squirt the food into his mouth.  You don't want him to choke on it or for it to go in his lungs, so start with just a little bit.  If you don't have a syringe, you can get one from the pharmacy.

With any kind of drooping, I think of stroke. How old is your cat?

He's about 2 years old. I think he feels better now, he ate normal last night and today but I'm still pretty concerned about the drooping lip because it may be something completely unrelated. How common are strokes in cats?

They are very uncommon in a young cat.  He's eating normally again which is very good.  I would take him to a vet for a check just to make sure everythings okay.