Oh gosh ! ..
Ok, don't panic. Spasms don't aggravate fissures that are healed. It is not too late. Tomorrow morning your BM would be fine, because you haven't stopped taking your Miralax yet. Don't get stressed.
Sitz bathing and applying nifedipine are the two best things that you did. It will help you tremendously to have a smooth BM next time. So please please please relax. Don't think about it. Let the cream work on your sphincter muscles. Stress causes the muscles to contract, so please, while having your BM tomorrow morning, try to exhale more. You will be fine. I will be praying for you.