I am always very sensitive to behavioral changes in my cats. It often means that they are in some kind of physical or emotional discomfort or stress. I first check out all the obvious things, like any unusual events that may have upset them, any part of their body that seems to be hurting, a temperature, checking to make sure they haven't torn a claw, the condition of their mouth -- cats often have mouth trouble, and just like humans, they hurt there, and they can have gingivitus, or even a broken tooth. You can also see from the color of the gums if the cat is anemic. Many times a virus will make their gums pale, and anemia as such will make them very pale to almost white. Also check for ticks, fleas, ar mites or worms -- all of which can make the cat feel like life is a bummer.
You can do this before you go to the vet. If the behavioral problems persist and there is nothing obvious, the vet should do a more serious screening (blood, urine, etc.). It is possible that there is some kind of problem with her reproductive organs. A viral infection. Blood parasites.
But why are you letting her have kittens? It would be so much better for you and for her health if you had her spayed.
I hope you find he problem. Sometimes these things pass. Of my three cats who developed bad tempers for a time, one had renal blockage from crystals in his urine, one had massive hairballs, and the other went off for a week and came back thinner and in good temper. So there can be a lot of reasons, and the vet is the person who can be most helpful.