Should i be worried about a new black beauty mark on the inside of my cheek?

Yes: This needs to be checked out by your dentist right away. The fact that it is "new" concerns me but it could be that it's been there a while and you just noticed it and that could mean something else. Black or blue colored lesions in the mouth are uncommon (though more commonly not a problem in certain ethnic groups) and need to be evaluated right away.