The problem with JW's is that they always want to be in control and they like to give the impression that you have no choice but to obey them. She has absolutely no legal (or even moral) Right to try and force you to go to the ASSembly if you're sick. Or even if you're not sick. This is not nazi Germany and if you do not want to go, you are under absolutely no obligation to go.
In reality, she was far more interested in the status she'll receive if she actually "brings someone into the truth". The fact that you no longer want to listen to their bullshit study, might reflect badly on her (or so she thinks) and that probably infuriates her.
If she bugs you again, tell her that you read Crisis of Conscience by one Raymond Franz and if she really wants to know why you no longer want to study, she should get herself a copy and read it. I guarantee you, she'll never bother you again.