I'm in my mid 40's now and this pattern is totally normal for me now!
Gone from being 25 days each and every month to now going 36, 33, 30, 18, 38, 20, 26, 45, 17, 33, 26 and 41 over the last year. Add those all up and I've had 12 periods in 363 days, which puts it into perspective for me.
That certainly is my age though!
At mid 30's I'd put this one down to being a blip. When I got married I was 25 days regular, which would have put my wedding smack bang between the two. Then on the day before the wedding, surprise!
Our bodies just love to confuse us don't they?!
Firstly do a pregnancy test, just in case, but assuming it's negative, keep a track of your cycles and if they start regularly going off track, I'd speak to someone at your GP surgery.
Mid 30's isn't unheard of with early menopause but most likely it's just a blip in your cycle.