Why did my cycle randomly change?:/

I always got my period every 29 days,I always had a 29 day cycle. Now two months ago my period randomly changed to 35 days.. Why? Is it irregular now? I don't take birth control and I find this extremely weird.. Now I wouldn't even know when I was fertile to avoid sex on those days.. 
I'm not on birth control because I don't want to mess up my chances of kids in the future and I didn't have a good experience with it last time. We would be prepared if we got pregnant but we are not trying. What's up with my cycle?

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