weird bladder pain

so i'm supposed to get my period tomorrow.... and right now, like all day& yesterday  i've been feeling this weird pain at my left top side of my bladder.. it's like this weird pulse.. it comes and goes it's uncomfortable & it kinda hurts
idk if it has to do with my period or what? is this normal? what is it??
it has never happened to me before 
what do i do 

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