Side effects hours after injection?

Hi!

So we found out last week that our cat has FIP. He was turning yellow already and we thought thats it.. luckily we get the diagnosis and immediately started the therapy. We are on day 6 now and he got better really fast. He is slowly starting eating again but still not everything like before.

Yesterday after the injection it seemed like he was very tired and was sleeping the whole day.. in the evening it got better. Today the same. The days before he seemed to be more active. Anyone know if this is normal? I get it that it must be very tiring fighting this thing but we are just hyper sensitive now after almost losing him last week.

Also is it normal that you have to push pretty hard to inject the medicine? Our vet said that we might not be deep enough and still in the skin but the needle is pretty deep already in my opinion.

All the best to the kitties out there!