If your cycles become irregular, what are the odds of it changing so much so quickly?

I was on the pill so I had the normal 28 day cycle, like clockwork, never failed. Now that I've been off the pill since like June, I've still been very regular. My cycles are more like 26 days though. My last cycle was 26 days and this cycle af should have showed up on the 18th. What are the odds that my cycle jumped from 26 to 31+ days in well just that, 31+ days? It's very possible I'm pregnant, but the hpt tests were negative, but it was still early I'm uming. I'll end up taking another tomorrow, I'm just afraid I'll give my hopes up because I've had a combination of symptoms but of course that doesnt always mean anything. (pains in my abdomen, back, and hips) I would love love love to be pregnant. I haven't been stressed or anything and I've been eating very healthy for the past month. What do you think preggers or weird cycle mess up?