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Any ideas what’s going on

So I am 18 days late on my period... never been this late ever I have taken pregnancy tests and I think I saw the faintest line on one but could have just been my eyes playing tricks on me and every other one has been negative or invalid. This week I’ve started to have heartburn and I’ve never had heartburn ever. Is it possible for me to be pregnant but it not be showing up on a test?