Helppp...

So me and my partner have been TTC && I have had okay periods and in December I missed my period... but on the 7-10 is when I usually get my period.. I had bought four pregnancy tests and they all came back negative.. but when I started my period in January I cramped very bad in my lower back (never happens) and of course the regular cramps but with a little more power and some oddly big blood clots... I'm not sure if I could be pregnant && not know or what has happened but this is odd. Any suggestions?