Many women seem to go through the same thing. I didn't, but my oncologist warned me I might. You have been very actively focussed on treatment, and possibly side effects and now there is a void. Your next course of treatment will fill that gap, and you will probably feel much less anxious as you are doing something positive. But it may recur once radiation finishes. The focus on treatment is useful because it occupies the mind. Without that, as you have found, you focus on the cancer and the impact of realising that you have had something that dramatically affects your sense of who your are, what your life offers and your own mortality. That's enough to make anyone anxious.
Part of recovery is coming to terms with that changed perception. There's no right or wrong way, you need to find the path that's best for you. Some need time out after active treatment, to think and reflect. Some take a long time to come to terms with that change, but that's because it takes time to feel prepared to do so. I had a few sessions with a counsellor - I wasn't depressed but a bit confused about my life and my expectations. It was extremely valuable. Give yourself some leeway and deal with one thing at a time. Treatment can be hard and demanding, fitting the reality of cancer into your life, even when you are cancer free, is a slippery devil and may take some patience. But you will. Best wishes.