Wondering how rare is it for an obese women to not test positive on a pregnancy test, but she is pregnant?

I seem to have a fundus right at my navel(20weeks). The problem is I keep getting light menstration each month. I have heard of obese women going to full term while still having their period. I have also heard of obese women not knowing they are pregnant and giving birth when they had no clue they were pregnant. I am going to 'educational health' through my HMO Tuesday, but I don't know what to do or ask. I am scared to ask or be assertive about this. I have had miscarriages before, but it wasn't established that I was medically pregnant in any of them. If I am pregnant, I want care so I won't miscarriage. This might be my last time to have a child. I don't beleive in abortion.