What Causes Delay In Periods,nausea And Uncertain Pregnancy Test Results?

Hi I have been having pregnancy symptoms I have nexplonon in and have had it sense 2012 I had regular periods antill now I havnt had my periods for about 5 months my stomach had become hard and round and I have been nauseous and really tired and my feet have been swelling I have taken 5 test all together 4 at home test of such one came back positive and the other three negative and I just recently went to the health department to be tested and that one was also negative but I feel there is no way I am not I just have a feeling what should my next step be