I DONT KNOW WHAT MY PLANT IS DOING!!

When in doubt, check the PH of your growing medium. If you don't have a reliable PH meter, you will generally be flying blind. Looking at the leaves, I would guess that the growing media has gone too acidic (low ph). You can buy finely ground agricultural lime to blend into your soil. Mix soil, wet/hydrate, let sit for a day, test PH, add lime accordingly, mix, test PH. etc. If your soil is way out on the PH, repotting with properly adjusted ph is possible, if the root mass can be gently teased out of the old mix.

well its hard to judege on basic of one picture, i have to see the whole plant to give some reliable advices. could be just an old leaf, or to dry or or or.... give more informations pls, you can hit me per dm

Make a diary and someone will help you

Dont you have more pics? For me seems like nitrogen toxicity, the leave até dark green. May an overfert by excess salt. Are you running coco/mineral or organic soil?