Female rottweiler 2yr old nervous barking help please!!

hi,

need help with my rotty. we have had her since a pup. never been stressed as far as I am aware.

But she barks and backs away around other dogs. seems more with males. she goes to training classes and even with dogs she knows she periodically barks.

if I tell her to stop she does almost instantly. We have watched her and there doesn't appear to be a trigger we or the trainer see.

I can walk around the class dogs on the lead and get real close with no issues. then she'll bark at one for no reason.

she's even been off lead with other dogs she has barked at.

we just don't know how to make her relax, and not be barking/ afraid around other dogs

thanks all for any help. if it wasn't for the barking she'd be an even more fantastic family member