OPK positive but no CM

Hi guys! This is my first post on here. 
The last 3 months I have been TTC I have regularly had 28 day cycles. I have noted my CM has seemed most "fertile" on day 15 and 16 due to Egg White CM.
I took an ovulation predictor kit test for the first time this month and yesterday (Day 8) around 11:30am I got a negative. Last night I got a smiley face at 9pm, so hubby and I got right to it.
The weird thing is yesterday was my 8th day on my cycle. And today I took another opk test and it was negative. Would my surge be that short? And would you trust an opk test over CM? I am so confused! 8 days seems so early. Thanks for any help!!

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