Confused about period?

Thank the lord. I had been stressing I was pregant. 13 days late and just got my period. At least I think it's my period. Usually my period is really really heavy when I wake up and red as shit. But this morning it wasn't a lot it was light but decent I guess did get on my underwear and was brown??? Having a bit cramps and usually I have insane cramps. My left shoulder was hurting so bad before this. ( never happened ) Is this my period?