Hi, there several possible causes of both straining to urinate or defecate. With cats, it can be sometimes difficult to tell one from the other, so your cat may be having difficulty with both urinating and defecating, or it might just be one or the other. If she is having trouble with both, it may be 2 unrelated problems.
Straining to urinate can be caused by irritation of the bladder from an infection or inflammation. Cats can develop a condition called feline urologic syndrome or feline lower urinary tract disease which causes bladder inflammation and discomfort. If you aren't finding any normal sized urine clumps, she may be urinating small amounts each time she strains. A blockage of the urethra from a bladder stone or mucus plug can also cause straining to urinate, but this is much more often seen in males than females. If your cat was blocked, she would probably also be much sicker and be in pain. Most females just tend to get the irritation and inflammation, and they will urinate small amounts frequently, often along with a lot of straining.
Straining to defecate can be seen with both diarrhea and constipation. A cat with diarrhea strains because of irritation of the colon which makes them feel like they still need to go even when they are empty. If she was having very small amounts of runny stool each time, you might not see it in the litter. Cats can also get constipated for a number of reasons, such as dehydration or eating something that causes a stool impaction.
A neurologic issue like an injury to the base of the tail
could potentially cause problems with both urination and defecation, but this would more typically cause incontinence, and not necessarily straining.
Especially since there are so many possible causes for your kitty's problems, I would recommend a vet visit so that a vet can examine your cat in person. Many of the possibilities discussed above require medical treatment to help your cat feel better. Good luck, and I hope your kitty is back to normal soon!