I am sorry to hear your guinea pig is struggling with her digestive system & doing soft poos.
This is our experiences, I hope some of it can help you.
One of our guinea pigs, Hamish,

(who’s no longer with us) struggled with his poos. The muscles in his anus were quite weak, the vets words were, “If he was a person I would say you need to increase fibre & do exercises!” Guinea pigs diet is naturally fibre rich with hay. We settled on 1 dose/helping of fibreplex daily. Which worked for him. The Vet was happy with our regime. Our Hamish was also on longterm, daily Metacam for pain at the base of his spine & a glucosamine supplement for possible arthritis.
Hamish was unique, he loved fibreplex & would happily eat it on top of a nugget like icing!
We currently have Herman pig living with us. He has multiple unknown, goodness knows what going on inside his little body! Herman is not on any medication.
The vet is happy with him, (he sees the vet every 12weeks)he’s lived with us about 7 months now. He’s a happy chap. When he does splatty poos, I remove all veg until they are better & reintroduce 48hrs later. We also increase the probiotic & offer him soft recovery food. Which seems to helps his guts, rebalance.
We have discovered my garden lettuce does not agree with him. Herman has no sweet food, and no sweet dried forage. He’s also on a lifelong ban from pea flakes after the rescue discovered they massively disagreed with him.
Herman hates fibreplex, I struggle to get him to take it any which way, so he gets one pellet of protexin pro-fibre daily which I have discovered is enough for him on a normal day.
The protexin pellets were also recommended to us by Herman’s guinea pig rescue.
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