Third sprint of a startup company with one developer? It should not surprise them that there are a few issues on launch. And that isn't a criticism of your development skills, even a startup should have hired a more experienced developer to give you guidance and offer second opinions on your coding.
Being the sole developer will have its benefits too. If your colleague(s) come from no development background, YOU are the one best placed to suggest setting aside a sprint to work on live bugs before any further work can be done. If they have been trusting with you up until this point, I would be confident you can make your case that time be spent on research and fixing bugs so that they don't become bigger issues in the future.
Given your description - especially of their 'full trust' in you - it doesn't sound as if they are looking to fire you. They may be aware of your inexperience and expect some growing pains. I've known some startups to take on relatively new developers as a kind of 'investment'; you might not have spent many years in the field but it will pay off in the long run for everyone to have you there from Day 1.