My cat had surgery a week ago and is now acting very lethargic. What should I do?

My 11 year old female cat had to have surgery last Tuesday (December 20th) for a lump located towards her left back leg. The lump was sent out for a biopsy and came back as a certain form of untreatable cancer. Since the incision was fairly big, she had to have a drain tube put in to help keep fluid from building up in the incision. She had the tube in for 3 days and then I had to return her to the vet to get it taken out. The vet let me know that the holes from the tubes would continue to drain, so there would be no need for me to worry. In those days her mood lifted and she was beginning to be her normal self. She was more active, her appetite went back to normal and she was being her cuddly, cute little self.

Today, she seems to be really lethargic. She hasn't been very active today and I haven't seen her eat or drink anything. She doesn't react when I pet her. In fact, she constantly looks groggy- Like she just woke up. I noticed her shaking a little today while laying on the couch and she acts like she doesn't even want to be touched, like it hurts to be pet. When I went to check on her later in the day I noticed she wasn't on the couch, instead she was behind it, laying down on a blanket. She's been hiding away ever since. When I go back to check on her she notices me and glances then goes back to trying to sleep. I've noticed that just putting my hand on her makes her tense.. which makes me think she's in some sort of pain. When I went to sit down she came out from behind the couch and tried hopping up a couple times but couldn't do it- And usually, she's able to do that with no problem since it's very low to the ground. Today, she struggled and right after trying to hop up, the holes where the drain tube used to be started leaking all over and she got blood tinged fluid all over a pillow she was standing on. Assoon as that happened, she freaked and went back behind the couch. I've been checking on her since. She's moved spots behind the couch but nothing else. She won't come out at all.

Is there a chance her incision could be infected and making her this way? She seemed perfectly fine yesterday and now today she's done a complete 360. I plan on taking her to the vet first thing tomorrow morning. Has anyone had any cats recovering from major surgery act like this? If so, what did it end up being?