Very scared and nervous... please help

So a little while ago I posted about being nervous that my period was going to coincide with my school swimming carnival (thank you for all of the advice that I was offered xx). Unfortunately I wasn't able to start my period early and the carnival is tomorrow... And I still haven't gotten it... At the moment I am hoping, begging and praying that it will hold off until Saturday, but there's always the chance that it won't. I was wondering if anyone has any advice about what to do, should my period start while I'm competeting. 

Some things you should probably know: 

• I have never once tried to insert a tampon

•I have swam once before with only a panty liner in (however this was towards the end of my period and the flow was very light) and all was fine

• My sister has swam right in the middle of her period (without a tampon) for about 20 minutes and nothing happened 

• I had my first period when I was 12 and 11 months (August of 2013) and have never missed one (or been this late - 4 days) PS I am now 15 and 5 months... 

• I have no clue as to why this period is particularly late

• My friends and I are more or less in sync and most of them are at varying stages of theirs right now, while mine hasn't started 

• I am very stressed and nervous about tomorrow, and would appreciate any help I am offered 😕😁😭😘