Congrats on your pregnancy. Sorry to hear you are having tough time, but do what you can to enjoy the remainder of your pregnancy and to make it as relaxing as possible. I would find the situation tough to stomach as well. What would be useful now is to think "ok, this has just happened, what can I do in the time I have left to reasonably lessen the blow"
First of all, put yours and bump's health and well-being at the top of your priorities. Don't consider amending your AL to accommodate this blip!
What tasks would be useful between now and your annual/ mat leave, in your opinion? For example, you could use the time you have now to just focus on writing up some handover notes/ handing over to your manager. Anything that you think would be useful for them or for whoever eventually picks up your work. You have several working days to do this, which is better than none at all.
Some maternity covers/ internal staff who pick up the extra work in their colleagues absence don't get the luxury of receiving handovers beforehand for various reasons. In those situations they just have to proactively figure things out for themselves.
Your manager might be relatively calm about it because they feel they have everything covered/ maybe they are secretly worried on the inside but don't want to worry you!
It's lovely that you care so greatly about your work, but it will/ can be taken care of in your absence. Best of luck and enjoy your time off when it arrives x