How to answer ...?

I've been having morning sickness like you wouldn't believe, I still have to go to work but needless to say it shows im not feeling my best. When co workers ask how I am and I just say " oh under the weather" or "im not feeling so hot today" they instantly say " are you pregnant" ? I have been saying no but I hate lying ... I hate more that they ask but ... I'm not ready to share .... How are you all dealing with this coming up ? And it's like if I hear it once I've heard it a million times ... " are you sure you're not " , " that's how I was when I was pregnant" ... Can't wait to just get it out in the open .