Kitten trembling, acting strange

So yesterday my kitten started trembling and rolling around weirdly. I wondered because she had been more vocal if this was her first heat. (She's not 5 months old yet, and I was planning on spraying her this month. But maybe Mother Nature beat me to the punch.) anyway, the trembling and shaking freaked me out though so I took her to the vet. They did blood work and everything came back normal except her WBC was low. Overnight she was crazy. She couldn't sit still, meowing, chirping, rolling around. I though ok. I guess this is heat.
Today, she is totally normal. As she is napping here by me, I saw a slight tremble again.

So QUESTIONS:
1. Is trembling normal? Is it a symptom of heat?
2. what about WBC? She was previously tested for fiv and felv... they took blood to test her again and I'll just wait and see...
3. Do symptoms of heat come and go in a day?!

It sounds like heat to me. She is on the younger side of going into heat so it could be an early heat or her body getting ready to go into heat and she is reacting to the changes.

I'd go ahead and get that spay scheduled. 

While she may be in heat, you do NOT want to schedule her spay until that WBC rises to normal levels.  A low WBC indicates she could be susceptible to infections, she could already have a viral infection of some sort.   Has she recently been on any medications?  Any flea control applications?  What was the vet telling you?  Did he ask you to come back in a week to recheck the WBC?

Depending on your description of "trembling" during heat...no, it's not normal.  Being overly affectionate, clingy, playful, rolling around, chirping are typical signs of heat - but, you have to talk to the vet about her low WBC count and his plan of action to address that.

No, no medicine at all. What I mean by trembling is just that-a small almost imperceptible shaking. Like what they do at the end of a really big stretch- but not when stretching. Like shivering. Today and yesterday she has been sleepy and hasn't eaten very well. I called the vet and left a message but got no response. I'm pretty unhappy with them at this point because I've gotten zero advice on what is wrong with her or what I should do.

Call another vet and try to get an appointment right away.    Whether she's in heat or not is probably redundant, you want to know why her WBC is low, and currently, why she seems off and not eating much.