Positve then Negative OPK

Typically I test 3 times through the day. Morning, afternoon, evening. Yesterday I had a negative opk at 8am but, missed my afternoon and evening test because I was gone and forgot to take them with me. Today had positive opk at 8am and negative opk at 2pm. I've also had a lot of cramping today. Could my LH surge have started last night and I just caught the end of it this morning? If so does the negative LH this afternoon mean I have already ovulated? This is my first cycle using OPKs and to be honest I find them more confusing than helpful.