TIFU by accidentally taking time off work

For context, I just recently started working a new job. After a few weeks they began to lay back on shifts because there were more employees that needed training. The following week I ended up being sick for the first day of that week, so I called in and said I wouldn’t be able to make it but I could head in tomorrow. The next day I was still rostered off, so I accepted it. This pattern continued the rest of the week, until I decided to try and contact work on Thursday. Unfortunately the phone service for both mobile and landline phones wasn’t functional that day, so I had no way of contacting my employer. The following day I was able to call in and ask to be put on shift next week. They agreed and then at the beginning of next week I got food poisoning. I received a call from Human Resources and apparently they assumed I had been sick all of last week and that’s why I hadn’t received any shifts. I tried to explain the situation but they said I needed a doctor’s certificate to prove I’d been unwell. So now I have to try and make a last minute appointment before heading in to work tomorrow and try and explain this situation to a doctor. I know this is more a stroke of bad luck and misunderstanding than what you normally see on this subreddit, but I’m panicking about this a bit and just needed to talk about it somewhere.

TL;DR: I accidentally took over a week off work by calling in sick for one day off, and now my employment wants an explanation.