So, the deal is you got your period while with the boys. Here is how they would react. 

Dallas: You're strolling through town with Dallas when he goes to check out your butt like he does. When instead he sees you've leaked all other your jeans, and starts to swear a lot. You stopped to see what was his problem now, to see him sweating with nerves. Out of no where he pulls your bottom into his groin to hide it. He makes you sit on his lip cause you now... he doesn't exactly put too much thought into it. Now you can't stand up because he has a red patch on his jeans. You're forced to wait until someone you know drives past to help you two out. 

Johnny: The two of you are hanging out as his place when you start to have violent cramps which feel like you're being stabbed a billion times over. Johnny completely freaks out because he has no idea what is happening, you manage to explain. He turns bright red then ends up running into the kitchen to get some pain killers. When the pain subsides you and him go eat ice cream on the roof.  

Darry: He notices you talking to someone, and comes right up to wrap his arms around you. He then whispers in your ear what was happening, it makes your skin turn pale. You've leaked. Lucky for you, Darry is very mature and goes to buy you supplies. He isn't embarrassed, and tells you not to be either. 

Soda: You were doing it with Soda, like always but this time he stopped. Soda had noticed there was some blood, you wince in embarrassment and try to clean it up. You promise him that you didn't know you were on it when he started... but he tells you it's fine and he'll lay down a towel next time. 

Steve: Lately you've been fighting a lot with Steve. The entire gang has no idea why you've been attacking him, and nobody else. Whenever you get around him he makes your skin crawl for some reason. You loved Steve for the longest time, and didn't know how to express it. Finally you had enough of everyone making fun of your moods, and announced to them, "Listen. I am freezing cold and on my damn period, so do not mess with me!"  Steve turns red, and hands you his jacket. 

Keith: He wasn't much for sappy movies, or any to be exact. He typically couldn't stay still for very long. You have a bad week, and he let you lay on his chest while you watched sad movies and cried a lot. He even went on snack runs. 

Ponyboy: You're at school when the class bully comes over to you. For some reason he has a crush and also can't take a hint that you aren't interested. When you tell him to get lost, he announces to everyone that you must be on your period. Ponyboy stands up in front of everyone and says, "So what?! It's natural and nothing to be ashamed about."