RE: how young can a female go into heat?
How old is your pup?
Females go into heat after they have reached adult height and weight, depending on the breed the age can range anywhere from 6months-24 month. Every dog, bloodline, breed is different.
If your pup doesn't meet any of these age specifications I'd be giving the vet a call just to be safe.
Is there any odor to the "snot"?