Um yeah I would be super creeped out if my sister started keeping track of my periods. Are you two in sync, and that's why she knows? That's the only way I can explain it beyond her rooting through your trash. It's weird.
But I would listen to your mom. Go home, take a PRN, maybe a hot bath....something soothing. I never had issues with PMS until my first manic episode; now I get super emotional and teary once a month. Hopefully it will pass. Mention it to your pdoc the next time.