Random cycle change

Cassandra
For as long as I can remember I've had 26-28 day cycles that last for 4 days. For the past 2 months they've been noticeably longer. Last month was 33 days and this month I'm at 35 days and still no sign of AF
Any ideas why this change? 
​I called my Doctor last month and they said it was probably a change in my routine. I haven't been sick, I'm definitely not stressed, I have not gained or lost weight, sleep schedule is the same. 
​Needless to say, I'm stumped and frustrated. I'm going on a trip this weekend for my 30th birthday and had hoped it would be done by now!