Inserting fingers into the vaginal canal can have a few different causes. One may be that you had less lubrication (vaginal or from a bottle) than you did the other times you've tried it, or you may have been more tense than usual. Heck, sometimes you can catch yourself with a fingernail and cause discomfort that way. Do any of those sound possible?
I was actually thinking about my finger nails but they're not really long. I usually get really wet when I masturbate but I was also thinking that might've been the problem too.
Do you have any ideas on how to get wetter without lube since I'm not able to buy any?
Generally speaking, the more aroused you are the more likely it is that your body will be producing enough lube to make things comfortable. So it might just be a matter of making sure you're really turned on before inserting your fingers. Too, you may be able to access certain kinds of lube without needing to buy them. If you check out this article, you can see what kinds of things can be used as lube and how to get them: Lube 101: A Slick Little primer.
I don't think I was aroused enough. Is there anything I can use at as a substitute lube since I can't get any of the things in that article. My parents can't know I bought anything.
Do you have things like massage oil, baby oil, olive oil, coconut oil, or Vaseline in the house that you could use? If not, things like Vaseline are sold as general lotions, so if you can get to a store to buy one it wouldn't alert your parents to the fact you're purchasing lube. Too, it may just be that, on days when you're noticing your not producing a lot of lube, you focus on external stimulation.