I am shocked at your boss for making this comment and I would take it high offensive to it.. Totally.... Slap her next time, only joking, but seriously, I would take her aside and ask what her problem is.
Being pregnant you are going to be extra sensitive etc and in no way shoudl you be treated like that. When I was pregnant, I loved my bump getting bigger and actually wore pregnancy clothes that fit around your bump and make you look bigger and I was very proud. If any woman or Man asked me to hide this bump, I would of got very upset....... I worked as a manager and had women and men on my team in a busy office meetin clients etc and never would I speak to anyone like this.
She - the manager obviously has issues but I would wear your clothes as normal and DONT hide your bump and if she brings this up again, I would have a word with her and then reprt it to senior management....... best of luck