What are signs of heat? Is my kitten going into heat?

My almost 5 mo. old female kitten is sweet and friendly, but not very cuddly. Today she got reeaal cuddly with me, drooling and all, and they she started biting my hand/arm, although still purring and stroking and wanted more cuddling, but then continuing to bite me (not very hard at all). She is normally not a biter at all.
Is this a sign of heat? I have an appointment for her and our 4 mo old female to be spayed at the same time, in a couple of weeks. I'd like to take them both in at the same time, but my vet doesn't want to spay too early. He wants to wait until the youngest one is close to 5 mo. and at at least 4 Lbs. That's why it's not been taken care of yet.
Will cats behave this way when just cuddly sometimes, or is it a sign of heat?
What are the general signs of heat normally? Can there be more suddle signs, or is it pretty easy to detect?
I can't find the search area, otherwise I would have done a search on that. Is it not up running yet?

Sounds like she is going into heat. You should find a vet whio isnt so old fashioned and get her done NOW. It is a lot better for her future health to be spayed BEFORE her first heat. You can have it done at 8-12 weeks or 2 lbs. Your vet needs to realize that she is plenty old enough, especially since she apparently is going into heat. Getting her done before she really starts her heat cycle for the first time will greatly reduce her risk of cancer and other disease when she is older. Tell your vet you want it done right away or find another vet.