There are literally hundreds of reasons (I know how unhelpful that is)

Every hormonal change, stress level change, all changes that effect you mentally and physically can effect this.

When in a new relationship mine tend to be further apart, I go through so many changed emotionally thats its bound to have a knock on effect somewhere along the line.

Also if mine become a little further apart for what ever reason I tend to worry about the usual (no matter how careful I've been) which ofc, makes them even further apart due to stress.

It should also be considered that although pubery has finished, settling into a period pattern can take a lot longer, and whilst most women have a monthly cycle, many do not, some go through it every 2 weeks, others much longer than a month.

Its all based on horome balance, and that changes a lot. I was put on meds because of a minor hormone inbalance a few years ago, it put my periods out from my "norm" by 3 weeks.

I suggest seeing a doc about it, like I said there are so many reasons, and worrying about it is literally going to make it worse.