what does it mean?

My cycles are usually 24-26 days, I was originally supposed to get my period on the 19th. I'm 3 days late. I I've been really nauseous lately, and had sulfur burps (smell like rotten egg) I've also been having really bad heart burn and lower back pain to a point that I can't get up from bed. I did have sex on most of my original fertile days. I took 3 pregnancy test prior to the 19th (expected period) and all came out negative. What could be going on?
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