My husband just finished chemo Friday, so it is early to tell if we will experience that, but from what i've heard, it's very common to feel lost and alone when you complete your treatment. You have had an ongoing support team of doctors and nurses who have been there for you through the entire ordeal. Then, all of a sudden, you don't have that regular support. Additionally, you have spent months working to fight the cancer and achieve this goal of completion. Achieving that goal is great, but there's also the worry that you aren't doing anything active to fight the cancer any more.
I also thnk that there can be a sense of loss after achieving any major lifetime goal. You spend all this time focusing on it and then don't quite know what to do with yourself when it's over.
I suspect that it will be a matter of time for you to adjust to life without treatment and to find your new normal. I'll be interested to see how things progress for you from here since my husband and I will be right behind you. Congratulations on completing your treatment!