Question about periods

Sorry if this sounds like a badwomensanatomy post. I had no sex Ed growing up (catholic school), my knowledge of my period was from the internet and peers. I'm 25 now so I've had a period for like 13 years and never had issues , except some irregularity when I was younger.

Well now I have a mirena IUD and like most women I don't get a period. Except for maybe 3 times a year. So my stupid stupid question. What happens to your period when you don't get it. Like where does it all go. Does it build up and you get a mega period? Do you get worse cramps if you only have 3 periods a year , because I feel that I do.