Why haven't I gotten my period?

I just had a baby two months ago (but sadly he didn't live long). After the birth I bleed for about 2 weeks. Then two weeks later I started bleeding again (I thought this was my period). Now, over a month after I had the bleeding, I have not gotten my period. Me and my husband are TTC. About a week ago I have some brown spotting in my undies. I have also been have pains in my lower abdomen. I have taken about three HPT but they have all come back negative. Could I be pregnant?