I just went tanning. like on my stomach and kind of thighs- i noticed that there were these like splashes of..

skin that didnt tan. my friend's skin is kind of like this. except its always been like that. i never really noticed mine like this. shes mexican so her skin is kind of darker than mine. not by much. do you think i can just notice it now cz i went tanning ? or is this bad? like ive seen stuff similar to this. i just dont know. and ive been tanning before. could it be like a birthmark type of thing ? i just want to make sure this is okay.