There could be many factors. It is recommended to see a doctor if you can.
But I can understand you may not want to.
It could be due to many reasons,
1. if you try inserting your fingers while not being aroused enough this could make it uncomfortable/possibly painful for you. When you’re aroused you naturally open up a bit making it easier to insert fingers or any other tools. So when you aren’t aroused, the area is tight making it hard.
2. Sometimes being nervous can make you naturally clench down there which could make it hard for you to put anything up there. A lot of it comes down to your mind.
I would definitely consult a doctor though, or even try speaking to someone older like your mum, a sister, an auntie, a cousin, whoever you feel comfortable sharing this with.
Hope this helps.