So I Have Been Extremely Exhausted The Last Few Days

So I have been extremely exhausted the last few days ..been nausea and puking the last 2weeks,my lower back has been killing me , I barley have an appetite.and have heart burn like no other.I had my period so I believe its my period a fee days early last 2 days..I have taken pregnancy test and they are negative I have had 2.kids and all signs point to being pregnant but,they are negative .so could it be that my hcg level are too low or what is your opinion that I should do??