I'm not going to cover my co-worker's shift
What will happen if you don't come in?
My co-worker will get in trouble, because he is not able to come into work today.
It is expected to help co-workers out if you are able to cover their shift. If you help them then they will help you when you need it.
I'm telling you I don't know if I will be coming in today.
It's understandable if you are not available to cover your coworkers shift. However, you should try to help out when you are able to. Helping your coworker now will encourage them to help you cover your shift in the future.