Ok I need someone's help!

I had a pos opk on 5/18, another pos opk 5/19 and at 5/20 at 2pm my last opk was already negative. I BD 5/17, 5/18, 5/19 and 5/20.  I been taking my bbt since 5/20 and it all ranges from 98.3F to 98.7F till yesterday morning 5/28. Today I woke up took my bbt before moving and it was 99.1F , my period is due on 5/31. Could this be implantation that made myBBT go that high ? My cycle is of 27 days . I took a pregnancy test this morning after my bbt but it was negative. Anyone has an idea ?