There's no rule about when to tell people, you can tell them whenever you want. You don't have to keep it secret till the scans. There are pros and cons to that approach.
I suffered terribly with sickness in the first trimester, causing me to have to fess up to work before 6 weeks. Within another week, I ended up having to tell family and two very close friends, as I had to cancel all social engagements.
I hadn't planned to tell before the 12 week scan, but had my hand forced. It's actually been a good thing because they've been very supportive through a few rough few months.
The only reason to not tell people before 12 weeks is in case you miscarry, and it's then awkward/upsetting to inform everyone. But I decided that there were a number of people in my life (close friends and family) who I would want support from, even if things went wrong. I was ok with those people knowing.
I haven't told work colleagues beyond my immediate boss, and I certainly won't be doing a Facebook announcement till closer to 20 weeks. But I'm glad I told some people early, and following my 12 weeks scan this week, I'm going to start telling people I see regularly who need to know (other friends and colleagues)
Do whatever you feel comfortable with, and if you need some early support, it's ok to get it. Yes it messed up your announcement plans (as happened to us), but it's not then end of the world.
Hope you feel better soon, and do talk to a Dr if the sickness gets really bad. You don't have to suffer.