Okay think of it a little bit like this:
When you are pregnant your work has to adjust your role to be as pregnancy friendly as possible. For instance, with mine I wasnt allowed to do heavy lifting anymore and was restricted to the office.
This is similar. Your work has a duty to protect you in your workspace. They can send you home and get you to work from there. If not, they still need to send you home and pay you.
As far as why we are 'suddenly' in the vulnerable category... well, we should have been because pregnant women have a weakened immune system as it is. However, with this being a new virus, they couldn't be sure. They have now had time to study it. They now have more cases involving pregnant women to use as a model for what advice they should give.
Now, they know if it can transfer to baby in utero. How contagious it can be. If women can breastfeed when they have the virus. They have more information and so are now telling us we shouldn't be putting ourselves at risk.
It's just the way it is. This is a pandemic. Look at credible sources for your information:
Government website
World Health Org
The Lancet
NHS
Keep yourselves informed. Be sensible and calm.
12 weeks sounds like a long time...but, to put things in perspective, wouldn't you rather be in for 12 weeks than pass something on to your newborn (I say that as a pregnant lass)? Wouldn't you rather be stuck indoors than endure a hellish flu that could potentially develop into pneumonia because of a weak immune system?