Question about OPK

I posted this over at AWH as well but thought Id post here too

I bought some OPK's off ebay and finally got them yesterday I tested anyways for fun since I don't really know what my cycles are doing, but if I go by my last cycle which was 33 days then I would have had to test on CD 17 and I was on CD19 yesterday so of course I got a negative result.

But here's what I don't get and maybe Im dumb: If I am on another 33 day cycle then my O date would be CD 20 right? So I get that testing on CD 17 make sense that you are about to Ovulate and that its better to have intercourse a couple days before you O, but then why would I get a neg result if Im about to Ovulate tomorrow(which is now today)..then that must mean that Im not Oing yet or that perhaps Im going to have a shorter cycle this month and I already did earlier..right?

Some women O within 12 hours of getting a +, so you could have missed it! Did you take it in the afternoon?

It was in the evening like around 5:30ish maybe 6

I would do another one now, but Im sure it will be neg, I have a feeling that if I did O, that I did already b/c I had been crampy for like 3 days in a row a couple days ago.

Let's hope I did!

From what everyone told me, taking it between 2-4 is the best time!

Well I took another one today at about 2pm..again still negative but let's hope I O'ed early!