BMI for your period to come?

hi :)

so my BMI is now 18, and i'm 14 years old. i've never had a period due to my eating disorder but the past few days i've been having discharge quite heavily as well as quite nasty stomach pains. i've been wearing TENA padding for aaaages due to my incontinence/anxiety problem so i've not had an issue with leaking...but i'm wondering - if i get my period, should i wear a sanitary pad over TENA? or use a tampon? or just TENA? i'm so confused! and is my weight high enough to menstruate?

thank you!

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