If your cat cannot produce any stool she needs to be seen by a veterinarian first to rule out the cause of constipation and have an enema performed if necessary. Book a visit today if you can, the earlier the better - if this were a dog he should be seen ASAP.
You are correct in saying milk is bad for cats. They tend to get an intolerance to it as they grow older which can cause GI upset like diarrhea.
Megacolon is a concern when seeing a constipated cat, so the veterinarian may request x-rays to see what's going on in the abdomen.
If your cat ate something that is causing a blockage, you may see a reduction in stool production, though typically with an intestinal block you will see diarrhea, vomiting, lethargy and anorexia.
Trond Hansen made some good points which I will elaborate on:
- Increase water intake. Water helps stools move along the intestines as the consistency is more moist and less dry. You can entice your cat to drink by having a water fountain (note: some cats do not like fountains), multiple water bowls and keeping food, water and litter box separate. When purchasing a water fountain make sure the pump is quiet, as some cats will be deterred by a loud pump.
- Adding more fiber to the diet: you can add canned pumpkin to his food in the meantime. Depending on the diagnosis your veterinarian may recommend a feline fiber food. I would hold off on the oils for now.
Again, it is very important that if your cat does not pass stools they need to see a veterinarian. Sometimes you need an enema or even an manual evac. With manual evacs the cat is sedated while the veterinarian digs/breaks down the feces through the anus. Some cats need surgery to remove feces as well, but that is by far the worst case scenario.