Why does my girlfriend have pain when i finger her?

Thats not my full question. She likes it and it feels good but after awile when she starts to shake and breathe very heavy, she wants to stop. I think she is close to getting an which shes never had before . she sais its hard to keep going. she had trouble explaining but she basically said its good but its such a good/weird/ painfull feeling that its hard to keep going. almost unbearable but not really painfull. im confused. Why/what is this. How can i make it less like this because we both want the to happen.? by the way, i mainly just rub/stroke what i think is her erus(?) which is up on the outer part of vag.