Hi! Please have her checked for mange mites. They burrow their eggs in the skin, which is very painful and not necessarily obvious in long-haired guinea pigs. The first sign is often that a guinea pig doesn't want to be touched.
Do not treat on spec with low dosed pet shop products, as you can easily make things worse rather than better. You will also always need to conduct a full three rounds course at the product specific interval once you have started mites treatment.
Long-haired guinea pigs can also find grooming painful, especially when you work on snags. You may want to consider keeping the bum end and the undercoat on the sides cut short to minimise matting and tangles. Short coats in summer are a relief for guinea pigs, otherwise they have to walk round with the equivalent of a skiing suit on a tropical beach. The hair will grow back by the autumn.
Gorgeous guineas long-hair 'posh n go' shampoo can help with minimising tangles as well.
Posh n Go Conditioning Shampoo 150ml
You may also find this guide here helpful once the behaviour, but only after you have sorted the other two aspects and made sure that she is otherwise OK.
How To Pick Up And Weigh Your Guinea Pig
" Biting" And What You Can Do