weird periods?

Lilith
Last month my period was over three weeks late, then when it hit it was like a damn crime scene and I couldn't walk (and I was super pms-y)
This month my period came a week ahead of what this app said, no cramps or anything letting me know it was coming and it's like the after period stuff, super light??
Can somebody tell me what's up because I have no idea and my doctors are stumped too because no birth control fixes this for me (my body doesn't react good to any type)