I think this post has listed quite a few reasons that really make sense.
I know that with several of my clients that I treat that have medical issues the part after treatment and during the waiting stage is very stressful. While you are in treatment,you have a feeling of taking action---of fighting back. When that ends, it often feels that you are just waiting to hear from others if you are ok or not. Talk about not being in control!!!
I know I experienced some of it after my husband initially finished his treatment. Of course,I was thrilled for him but there was some discomfort at not doing anything to fight the cancer.
For me, that feeling has pretty much gone away and we're just really enjoying the breaks between medical stuff where you just get to live your life again.
However, some of my clients tell me that their anxiety stays elevated during the in between periods for quite a long time leading to a lot of long, restless nights.
I think it's very important to let your MD know that you are experiencing this. There are many behavioral and medical ways to get help to sleep.