Missed period

Victoria

Hi I haven’t had sex for 4 months and I noticed that ever since I’ve gained weight my period has become irregular. Now that I’ve lost 20lbs and workout 3x a day I missed my period this month. I’m 19 about to be 20 and wen I asked my mom about she said it’s normal for a girl to miss a period and it not be due to pregnancy I just want to know if that’s true?

Thank you :)