Super Bowl sources tell TMZ ... most of her backup dancers -- upwards of 280, as we came to find out -- had no idea RiRi was preggo before she hit the stage this weekend, and that's due in part to the baggy clothing she'd been rocking during all the rehearsals.

Our sources tell us Rihanna was wearing very loose-fitting outfits both on Saturday and on Sunday as she was practicing her set. It wasn't until showtime that anyone involved saw her final costume, which seemed designed to show off her belly.

In other words ... Rihanna covered her bases here, not letting anyone involved with the halftime production get a whiff of her pregnancy before she could show it off to all.

Remember, Rihanna was also seen publicly a couple days before the Super Bowl, when she did an interview talking about the different setlists she'd created ... and there, too, RiRi seemed to be trying to hide her bump -- wearing a big furry jacket and other fluffy garb.

Of course, you could even trace this cover-up back to the Golden Globes in January ... where Rihanna and A$AP Rocky showed up in person, with RiRi again wearing a loose-fitting outfit and keeping her purse in front of her stomach throughout much of the broadcast.