Had 3 negative hpt and a blood test at 13 weeks which read I had a hcg level of less than 0.1. But I've missed my period again and getting the odd cramp now which would 16 weeks, what are my chances??

Neg pregn test . At 16 weeks you should look pregnant meaning your stomach should be protruding -is it? There are causes of negative HCG see below http://www.medscape.com/viewarticle/778183_3 If you are missing your period check thyroid , check prolactin and get an ultrasound to check for ovarian cysts and polycystic ovary syndrome.