Maybe it's not a trap set by enemies, but rather an old self - defense mechanism of a long - lost village. The cave might be set to protect something precious within, and Naruto's presence accidentally triggered the defenses.
One possible fanfiction could be about Kushina finding Naruto in a situation where he's lost in a forest. Maybe she uses her unique chakra sensing abilities to track him down. It could be a heartwarming story of a mother reuniting with her son.
One possible scenario in this fanfiction could be that Naruto discovers an ancient and mysterious shrine in the forest. The shrine might hold some sort of power or secret. Maybe it has a connection to the Sage of Six Paths. Naruto, being his curious self, would likely start to explore it. He might find strange markings or artifacts inside that he has never seen before.
In many fanfictions, Naruto might use new jutsus or a deeper understanding of his own power. Maybe he taps into a more powerful form of the Nine - Tails' chakra and combines it with his Sage Mode in a unique way to overpower Pain.
One common plot is Kushina finding Naruto in a different time or dimension. For example, she might end up in the future where Naruto has grown up and she gets to see the kind of person he has become. It's an emotional and interesting concept as she gets to witness all his achievements and the challenges he has overcome.
Yes, it could be a real story. There are many experiences in life where pain and gain are closely related. For example, athletes often endure a great deal of pain during training but gain excellent physical condition and the possibility of winning championships. However, without more context, it's hard to be completely sure if it specifically refers to a real story.
In some fanfictions, Naruto's pain might manifest through his actions. For example, he could become withdrawn and isolate himself from his friends. This is because Naruto has always been a social person, so any deviation from his normal behavior could be a sign of his internal pain.
One possible cause could be his sense of failure. Naruto always wants to protect his friends and village, so if he fails in a mission or is unable to save someone, it can cause him great pain.
You can try websites like Fanfiction.net or Archive of Our Own (AO3). On Fanfiction.net, you can simply search for 'Kushina finds Naruto' in the search bar and it will show you a list of relevant fanfictions. AO3 also has a large collection, and their tagging system can help you narrow down your search to find exactly what you're looking for.