Period talk !! Help

So I’min year 10 and on Wednesday, for class we’re going swimming at a beach, I need some advice for two things, first of all this apps says I will get my period two days after swimming but last month my period came 2 days earlier, second of all, I wear a little bit of makeup just to cover up pimples and make my skin brighter as well as mascara but I hate myself without it.

And also with the whole period thing I can’t really get out of it because we have to do the swimming because it’s too see if you can swim well and stuff.

Please please help xox