Her parents told her many times do not contact me in anyway, and I don't know what they will do to her when they find out she has been secretly dating me for a quiet time.
I suppose they could tell her to move out of the house because even though she is 21 they don't approve of the two of you dating.
Disapproval and distrust are a part of the witness construct. As you have already found out it is used to reinforce a high control belief system that does not encourage making friends with or dating 'Worldly' people which is you in this situation.
In fact they don't even want younger JWs to date....... they must see one another in groups. If you date your expected to get married. And of course you will not be allowed any sexual leeway, like fondling, touching intimately etc. before hand. If first base is a kiss that's it there are no more bases available. In fact if either of you happen to sleep over in the other's home and parents or siblings are away and your seen exiting the other person's house they will be DF as they will not accept that one of you slept in a different room or on the living room couch.
She is now suffering their disapproval. Should you and her elect to leave the religion in the near or far future you both could easily be shunned for the rest of your lives.
Should you have children and later divorce and leave the religion you could be shunned by your children if they in turn get baptized.
There is a lot to think about because it can get complicated.
Now as a born in your girlfriend may be going through the motions to keep Mom and Dad's approval. If she's a typical born-in JW all her friends are in the 'truth', a lot of her family especially the parents and siblings are in the truth. This being a high control religion being controlled........... is all she knows......until now.
On the other hand she may not agree with a number of beliefs and can admit she's in it for the people.
For both your sake's think about these things and talk about it together. Your relationship has hit a crisis, not of your making, but now she has hit a wall with her parents. How you resolve it together will be important.
Perhaps you should simply announce your engagement and plan to marry in the very near future. As a legally married couple. She is not going to get DF. If the parents slam her over this well she is 21 and has the right to marry the person she chooses. Put off having children for a couple of years to make sure you are committed to one another apart from any beliefs or non beliefs.