Any new veggies need to be introduced slowly as they can upset the tummy. Too much veg or grass on an unprepared tummy will cause soft poops and diarrhoea.
Please stop giving all veg immediately as her tummy now needs time to settle. She needs lots of fibre to rebalance the guts, so now and needs to eat lots of hay.
As hay is the most important part of the diet and you cannot judge hay intake by eye, so only way you know that a piggy is eating enough hay is to weigh them every week. Weekly weighing forms part of routine care. However when a piggy is unwell, you need to switch to weighing them daily so you can more closely monitor the hay intake.
If she is not eating enough hay or not eating hay at all, then you must step on and syringe feed. Syringe feeding directly replaces the fibre intake that she is not eating for herself. You will need to aim for 60ml of syringe feed in a 24 hour period (as they can often take a little at a time, then this may mean you need to feed every couple of hours.
Weighing her daily will ensure you can monitor you are getting enough syringe feed into her until she feels better and starts eating hay for herself again.
If the diarrhoea is severe then it is an emergency and she needs to see a vet straight away.
You are right, iceberg is not recommended but any other type of lettuce is fine. Spinach and other high calcium veggies, such as kale and parsley, need to be kept limited to one small amount once per week only. Too much calcium can lead to bladder stone formation.
Veg can only be reintroduced once her guts have settled and she is eating hay properly for herself and maintaining her weight. You must introduce veg slowly and starting with a herb such as coriander (cilantro) is best as they tend to be more mild on the tummy,
Not Eating, Weight Loss And The Importance Of Syringe Feeding Fibre
Complete Syringe Feeding Guide
Digestive Disorders: Diarrhea - Bloat - GI Stasis (No Gut Movement) And Not Eating
Is she a single piggy? If so, then please look to get her a friend when she is feeling better. Guinea pigs are highly social animals and require companionship from another guinea pig at all times, they must never be kept alone. Having a companion is one of the five fights of animal welfare.
The best way to find a friend is via dating at a rescue centre. Character compatibility is vital to a successful bond and if the piggies can choose each other, then you can be sure of that compatibility. Buying another piggy from a pet shop comes with a risk as you don’t know if they will actually like each other.
Bonding and Interaction: Illustrated social behaviours and bonding dynamics
Sows: Behaviour and female health problems (including ovarian cysts)
A Closer Look At Pairs (Boars - Sows - Mixed)
Companionship
Single Guinea Pigs - Challenges and Responsibilities
Regarding treats at a local pet shop - what is it you are buying for her? A lot of commercial treats from pet shops are inappropriate - they often contain a lot of sugary ingredients which aren’t suitable for them (too much sugar in treats or from fresh fruit or carrots can cause a gut imbalance and make them poorly) or contain seeds which aren’t safe.
You don’t need to buy treats for them - most piggies are happy with a paper bag (with the handles cut off) with some hay and a few veggies hidden inside.
Long Term Balanced General And Special Needs Guinea Pig Diets
Edible And Forbidden Veg And Fruit List With Vitamin C Grading
Enrichment Ideas for Guinea Pigs
Please do read all the green links I have added throughout as they go into more detail than we can go into in each reply
