Regarding milk fever (original question), the symptoms I noticed were panting, increased vocalisation (she kept coming over to me and meowing), weakness (failure to jump on surfaces), hyperventilation (over 100 breaths per minute), and sweating. Her muscles seemed to twitch a bit when she was trying to relax. She was also staying away from the litter during most of the first day. It was within 24 hours of the kittens' birth. I immediately searched online, trying to find out what was wrong with her and found the information about milk fever. I then decided to post here.
As Yvette mentioned in her comment, cats can die from milk fever. I also found this in my research online, so I was very worried. However, I couldn't afford to take her to a veterinarian, and I wanted to do everything possible from home to help her.
It's worth mentioning that she was the runt of her litter and her mom abandoned her in front of my home around when she was around 4-5 weeks old. My parents had saved her, but she was such a tiny kitten that she fit in one hand. Even to this day, she's about two-thirds the size of a normal cat (smaller than every cat in the neighborhood, in fact), and she's fully grown! So, naturally her health is a concern for me.
Allerleirauh's answer revolving around calcium helped me think along the lines of nutrition. I called the local pet shop and asked about food for nursing mother cats. They referred me to this brand of food for mother & kittens:
I was also told to buy a few of the wet, canned food (same brand) and to mix the wet food with the dry food, to make the dry food more appealing for her.
After 24 hours (she ate 3-4 times) I noticed her breathing was much better. She was no longer sweating and she was spending much more time with the kittens.
Today, she seems perfectly fine! She spends most of the day with the kittens and in the evenings she even goes outside to take her breaks in the back yard. She's very attentive to the litter and only leaves when the babies are sleeping. She's with them 90% of the time and never gone for more than half an hour, and she's no longer weak. She still gets a bit of a twitch, but I'm starting to think that's just normal.
Special thanks to everyone for their feedback and comments! It led me to find some nutritious food for her during this delicate time, and I believe that was the answer all along (previously she had been exclusively eating Friskies Seafood Medley).