Jack Russell advice, diarrhoea_tumors!

Hi,
I'm new to this forum. I have a Jack Russell bitch that is around 11-13
yrs old, age not 100% know because i got her from a friend & was from
someone else prior to that.

Last year she had a mammary lump removed that was a the size of a small
marble, she had other very small ones also, millimeters across, also
removed.

She also had a hysterectomy as this prevents hormonal probs that also cause
mammary tumors & pyametria at later age, the hysterectomy was at an advanced
age!

Sometimes she has spouts of diarrhoea & doesn't eat much, she doesn't eat
biscuits & hasn't much fibre in the diet, what can i do, i can't force
biscuits down her, although chewsticks, she likes! Mixing biscuits with
Cesar doesn't work!
She still sniffs around on walks, but seems a little uninterested .
When the upset tummy happens, her tummy rumbles a lot & makes bubbling
noises & she seems depressed, it's happened before & goes away, but can
anyone give me advice, as i'm worried that there may be a tumor link.

The vet says these small lumps less than the size of half a pea, are ok &
should be leaft, but does anyone have experience of this with Russells,
Russells seem to sniff round a lot & uses her tongue (which hangs out a
little, & always has) to sniff, mabe she has picked up an infection. Any
advice would be gratefull.
Thanks
Gary