False Positive? With hypothyroid on synthroid

I am 3 days late, I have hypothyroid, I am regulated so this is weird for me. I think I got so small craps but idk if it's in my head because I usually do get all no cramp pain till I acually get in witch I crap loads. I took a test and got a negative, only I have low hormones sometimes because I don't always take synthriod like I should. Whys your option? 

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