Why does using a tampon hurt? I need to know how to make it not hurt for when I'm camping. Girls only, please!?

I've had my period for about a year now. I've always used pads and I don't go swimming much so I never really had a reason to wear a tampon. Well, I had my period recently and I was at my friend's house. She tried to help me learn how to use a tampon and had me try it because we were gonna swim in her pool but when i tried i couldn't get it in. It felt like skin or something was blocking the hole. I couldn't get it in and gave up because it felt a little weird and started hurting a bit. The reason i REALLY wanna and need to try again is because i'm going to a camp to be a counselor in training and i could possibly get my period while i'm there. There's a lake and some swimming/lakeside activities and i don't want to miss them. Some are optional, like swimming or canoeing, but eventually i will have to swim there. There are girls and boys. If it was just girls, i think they would understand, but these guys are super jocks and b-tchy. They'll make fun of me if i don't swim or tip a canoe. I want and need to be able to use a tampon. I AM putting it in the right hole and i tried to relax but it just wouldn't go in. I can't even put like a pinkie finger in! There is a possibility i won't have it while i'm at the camp, but if i do i wanna be prepared.