If it's bloat then her sides will feel hard, like a very well blown up / overblown balloon.
And you won't see her chewing the cud, nor will you see her rumen working (the 'rumen wave' every 30 seconds or so) nor see/hear her belch. If it
is bloat, she'll become increasingly uncomfortable, and may even start kicking at or head-butting her stomach.
Chances are she's just got a gutful of grass making her tummy look big

Don't let her have too much cake in one go, but if she's just getting a handful at a time, that shouldn't cause her any harm.