Mid-cycle spotting, negative OPKs

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I was hoping maybe someone could give me some ideas as to whether or not something might be wrong. I have regular cycles, 31-33 days. For the last three months, I've had light spotting for 2 or 3 days in the middle of my cycle. I previously attributed this to ovulation spotting, but this was my first month using OPKs and I'm cycle day 21 and no LH surge! It's been 5 days since I had my spotting for the month. Anyone with a similar experience or have an idea of something that might be wrong?
I've been using clearblue digital ovulation tests, testing once per day about 4-5 hours after I wake up. Testing since day 14.