I imagine this has been covered somewhere in the long history of SDMB; but a quick search revealed nothing.
Anyway, I took a crap this afternoon - and man did I ever have a hemmeroid flare up. I was gonna take a shower but i was just like, “I’m gonna go lay down for a while tip the burning subsides.”
Now my cat is usually cuddly - i’s a very social cat, but this time when I went to lie on the bed she made different noises, and seemed more serious- like she was concerned, and not just after the deep tissue back massage I give her when she comes and cuddle next to me.
It seems that many people on the net are convinced of their cat’s concern for their well being - but I was just wondering what more critical observers would think.
Of course not! If you want a sympathetic, understanding animal get a dog!
My cats can tell when I’m not feeling well, physically or mentally.
Mine also can tell when I don’t feel well. If I have a migraine, they all sleep with me, and each cat has his or her “place” on the bed. When I’m just being a lazy bum, they take turns sleeping in the bed.
It might depend on your relationship with the cats.
My cats definitely know.
For the past month I’ve been experiencing a much-higher-than-normal degree of pain, and it didn’t take long for my two kittehs to catch on. When I’m walking, they’re no longer underfoot, but travel along with me at a close distance. When I’m sitting, they tend to stay near me, but not so near as to be a PITA. When I’m experiencing intense pain, they’re very unobtrusively nearby, just being there, and nothing more. And when I’m asleep, they let me sleep.
I’m wondering how long they’ll take to return to normal when my crisis is over.
My big orange guy is pretty empathetic. A few years ago when my senior kitty was sick, he stuck to her like velcro only leaving to eat and pee. When she died, I was devastated and he stuck to ME like velcro. After I had surgery last year it was the same thing. He’d also go complain to my SO and mom then come back to me.
I’m sure your cat knows.
I am unaware of actual studies on this.
Anecdotally, I think that cats (some more than others) do seem do notice when their humans aren’t emotionally or physically well.
I also suffer migraines, and notice that my cats (2 of the 3) have assigned positions, and will come and cuddle up close when I’m having a bad headache.
A few years ago, my daughter was having a bad day. She was crying and upset about something (time dims the details). Sprite, our more “motherly” cat, was insistent that she had to get into my daughter’s room. So we let her in. A short while later I hear, “Go 'way Sprite! I don’t want to feel better!” I go into the room and the cat is the on the bed with my daughter. My daughter is still upset and sniffling, and the cat is buttting her head against my daughter’s chest and purring and purring and purring. A few more “Go 'way!”/“purr” and the crisis was over. I don’t remember what my daughter was so upset about, but I do remember the cat was bound and determined that it was time for her feel better. 
A warning definitely needs to be issued for this statement. 
They can tell, they just don’t give a fuck.
Absolutely cats can tell when you are not feeling well. They hang close by during those times so that, should you croak, they can get the meat while it’s still fresh.
I think they do give a fuck - they know who puts the food in their bowl . . .
They can certainly read human body language - anyone who has attempted to take a cat for a vet appointment knows that: cats can tell you are up to something and make themselves as scarse as possible.
Whether they care or not about your pain is up to the individual cat … 
I had a cat who seemed to know. He would sleep directly on me most of the time, but if I had a migraine, he would come and just be near me. I found it very comforting (and appreciated not being touched). We called him Nurse Whitey.
I agree that they may not care, but cats are opportunists and they know who feeds them. If their food source is on the fritz, THAT may concern them. lol j/k. But it was there…
Bosco, the cat I had when my boys were little, spent most of his time avoiding my two little active and curious (and none too gentle) darlings. But whenever one of them was coming down with something, Bosco would let me know before I could figure it out by myself. He’d shadow the child in question and sleep in his room - something he never would do ordinarily. He was clearly tuned in
With a dog literally everything is smell. With a cat, it’s all sight and sound. Cats are very sensitive and acute to body language, posturing, and vocalizations. Both in terms of other cats and humans around them. And if you’re a cat person you become very aware of theirs as well.
So yes, this is SOP for cats who live around humans.