Weird cycle

So I have always had a super regular 28 day cycle. moderately heavy flow since I was 13 I am now 21. But now my periods just seem to come later and later spaced apart. My last cycle was about 40 days apart.

I am in nursing school, working part time and just got a puppy, I am really stressed out. But i also have never had a late period due to stress until starting in August. Any thoughts on what this maybe or probably just stress?

Thanks in advance.