The information in this answer is general; I have no specific knowledge about Florida.
The question is:
Do I have the copyrights to post a picture I took of someone on instagram?
The question body breaks this down slightly:
do I own the copyrights of the picture, and can I post it?
Neither copyright question quite makes sense. It should be the singular “copyright”, and there is no such thing as “copyright to do something”: you either own the copyright or you do not. (Copyright can be licensed, but this does not seem relevant here.)
In this case, yes, you own the copyright. This is generally the case unless you take the photos while working for someone else, or you have a contract that states otherwise; neither seems to apply here.
Then there is the question of whether you can post it. Of course, as the copyright owner, you can. But there may also be other reasons, besides copyright, that you cannot post it; these need to be considered separately. These reasons include personality/publicity rights and intimate image laws. They also include Instagram’s terms, to the extent you post on Instagram (or other Meta-owned services, which have similar terms).
All these issues are covered in more detail by other answers.
The question is also tagged ethics, suggesting that it requests information on ethical as well as legal issues.
With the above in mind, here are some other issues with the question:
A girl messaged me on Instagram
This makes it sound like the subject was a client, and you should not distribute photos of the subject at all without the subject’s consent.
to take boudoir pictures of her at her house.
This makes it sound like the photos depict the subject in an intimate way, and certainly in a place not visible to the public, which reinforces the idea that you should not distribute the photos.
I … sent them. She said she loved them,
This reinforces the idea that the subject was a client.
but she asked me not post the sexy ones
Now it gets confusing. This suggests that the subject expected you to distribute the photos after all, but why would the subject expect this? It seems like part of this story is missing.
because her boyfriend found out about the pictures and wasn't happy about it.
This is really the subject’s boyfriend’s problem, or the subject’s problem if the subject wants to respect the boyfriend. It does not seem relevant to this question. What matters is that the subject asked you to not distribute these photos.
I told her I would not post them immediately because I never do anyway, and when I post I will not tag her.
And…? Was the subject happy with this response? It seems like another part of this story is missing.
A few months pass by, and I post one of the pictures, and it seemed fine.
Presumably you did not tell the subject about this post, and created it in such a way that the subject was unlikely to find it (you said you would not tag the subject). Wrongdoing always seems fine until you get caught.
But it had a nip slip
Presumably the significance of this is that the subject objects to it. This does not seem relevant …
which I didn't notice
… and normally this would not seem relevant either. But together, they make one wonder: What other objectionable things did you not notice? This further reinforces the idea that you should not distribute the photos.
until she flooded my inbox asking me to take it down. Now both her and her roommate are spamming me
If you were not supposed to post that photo (and there is a lot of evidence suggesting that was the case), and the subject asked you to take it down, that does not seem like “spamming”. This attitude is worrying.
saying I don't own copyrights to the images since I never paid for the shooting
This seems to be based on a common (according to my experience on Wikimedia Commons) misunderstanding, namely that the subject of a photo owns the copyright to the photo. In fact, the subject has no rights under copyright law and the photographer owns the copyright (unless an exception applies, as discussed above).
Finally, here is a general comment: even if there is more to this story, and we conclude from that that you have the right to distribute these photos, that does not necessarily mean that you should do so. In this case, the subject’s removal request seems to have been made in good faith to protect the subject’s interests, by hiding body parts or poses that the subject would not want anyone to distribute photos of; you might gain more than you lose by honouring such requests.
I almost forgot: I am not a lawyer. But surely you knew that already.