engagement present from my fiance who's my dream filly.
That's a lovely way of describing her... & she deserves it too obviously, buying you a gypsy horse!!
I'm not sure with the thread title, exactly what you're concerned about, given the trailer trip was 2 days ago(?), so I don't think you have to worry about future effects of that. If she's off her feed or water or otherwise acting disturbed, safer to call the vet perhaps. If it's just that she's got loose stools(loose or actual diarrhoea?) & doesn't drink much, I'd keep an eye on her & look for an answer to the poos, but not stress.
Bran. Depends what you're feeding it for & why, but if it's wheatbran it can be a problem if fed regularly, mostly due to Ca/Phos ratio. Grain. What grain does she get & why? As above, it depends, but can have good, bad or negligable effects. A healthy, low starch, high roughage diet, nutritional supplement to balance nutrition and healthy feeding management(eg little & often, free choice hay..) will help her gut to become healthy & not 'prone' to colic. Probiotics - yoghurt - is a good measure too IMO. Depending on the situation/feed/problem they can be fed ongoing, or fed to aid the gut to get over something.