My period has been excessively late twice now

Hi,

I'm 18 year old.

I had sex for the first time on the 21st of June and ever since, my period has been excessively late.

It use to be every 28 days but now I'm looking at cycles between 45 to 50 days.

My last normal period (before I lost my virginity) was from the 14th of June til the 19th of June.

Then my next one came extremely late, on the 3rd of august and lasted about the usual 5 days

Then the next one, the last recent period I had, was late again and came on the 18th of September!

Does anyone know what could possibly cause this. I don't know what to do