Well said...birds don't go into "heat", but they become hormonal as puberty hits. Her rubbing her vent on your hand is a hormonal behavior, as they do masterbate. When she does this, please put her back in her cage or just down for a while, as this will only serve to frustrate her, and encourage her to continue to do it, and possibly to start laying eggs, which you don't want.
Nest boxes are only for birds who you purposely want to breed, as the nest box, or any type of box or similar container that is dark, only serves to make them more hormonal and want to lay eggs. So obviously you don't want that. Not only is your bird too young to breed, but you have no male, nor do you have any experience breeding or hand-feeding baby birds. So I don't advise that you get a male conure and breed her, and you certainly do not want to put anything like a nest box in her cage or allow her to be anywhere near one, as it will only make her more hormonal and start laying eggs.
Female parrots can lay infertile eggs in large quantities with no male bird present, and this causes the calcium to be leeched from their bodies, and it also risks them becoming egg-bound, which is fatal 100% of the time without medical intervention. So the best thing you can do is to discourage all of her hormonal behaviors and make sure that she has no small, dark places to go into/under, like a nest box, a cardboard box, under the couch, etc.
I see. Thank you for the informative response. I've been putting her on time outs in the cage each time she's rubbed.
I liked the idea of having an additional baby or two. Plus I had bought a large cage, but her previous owner included the cage - which I wasn't expecting. I wasn't well aware of the egg-bound risk. Good thing I didn't buy a box at the pet store today!
I don't encourage you to buy a male conure and actually breeding your bird, for 2 reasons:
#1) Usually when even a hand-raised, tame parrot is put with another parrot of the same species and the opposite sex, those 2 birds bond closely with each other, and they lose all interest in being bonded or having anything at all to do with humans. So if you bought this conure to be your pet that you can handle, who loves you, etc., then buying her a male green cheek conure will more than likely end that relationship. This is why "breeding pairs" of parrots are sold as "breeding pairs", meaning they are not pets, not friendly or able to be handled. There are rare, very rare exceptions to this, but 95% of the time you lose your pet.
#2) Breeding parrots is nothing at all like breeding dogs, cats, rodents, not even like breeding poultry birds or game birds. Yes, you can decide since you have no hand-raising/feeding experience to simply allow the 2 parent birds to raise/feed the babies instead of pulling the babies from the nest box between 2 to 3 weeks old, like parrot breeders do, in order to hand-raise/feed them yourself and to make them into very tame pets. In fact, it's possible to allow the babies to live in the nest box and be fed by the parents, and then simply remove them daily to handle and play with them in order to try to tame them. HOWEVER, YOU MUST BE TOTALLY PREPARED TO TAKE-OVER HAND-FEEDING/RAISING THE BABIES AT ANY TIME, FROM DAY 1 OF HATCHING UNTIL THEY WEAN BETWEEN 8-13 WEEKS OLD. Parent birds often just stop feeding their babies, they literally kick the babies out of the nest box, they start to pluck the babies, hurt or try to kill them, etc. So at any time you might be required to pull the babies and take over, any time from day 1, when they are teeny-tiny, blind, and extremely hard to hand feed without aspirating...most bird breeders have years and years of experience hand-feeding and have either been taught or mentored by another long-time bird breeder. Plus, you have to keep a Brooder on-hand to keep the babies in, as they cannot be out in the normal, ambient temperatures until they are fully feathered (~8 weeks old), you must have the hand-feeding supplies on hand, such as formula, syringes, thermometers, and all of the supplements required to add to the formula in case something goes wrong, etc., and you must be available to feed the babies starting out EVERY 2 HOURS, INCLUDING OVERNIGHT, FOR THE FIRST 2 WEEKS, THEN EVERY 3-4 HOURS UP UNTIL 4 WEEKS OLD, INCLUDING OVERNIGHT...only at 4 weeks old can the babies go longer than 6 hours without eating, and at 4 weeks old it's still every 4-5 hours, and so on. It's exhausting, and it's also heartbreaking because a clutch of 7 babies can turn into a clutch of 1 very quickly due to 100 different reasons.