LH levels fluctuate throughout the day. Most women's LH will peak in the afternoon, so it's common to only get positives then and negatives in the morning and at night. My darkest OPKs are around 3pm and are MUCH lighter at other times of day. It sounds to me like you probably just had a short surge - though I'm not clear on whether you had 2 positive OPKs over 2 days or just one? What CD are you on currently and how long are your cycles usually?
I would personally just BD every 4 days starting immediately, for the next 1-2 weeks, if I wanted to be sure I didn't miss O this month.