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OPK HELP!

So CD 13 I took a cheap opk and a digital Clearblue and cheap was negative and Clearblue was at "peak" I continued to check with cheapies and they all were negative... this morning I took both again CD 18 and cheap was finally positive and clearblue was also at another peak... so when did I ovulate? CD 13-14 or now CD 18-19??? My cycles are usually 28 days long