HCG up but nothing seen in womb

Just putting this out there to see if anyone else had this happen or something simliar.  
I found out I was pg at about 4 weeks. Within a few days, I did bleed and passed clots.  We truly thought that was it.  HCG has been 73, then only 90 48hrs later, then still 90 48hrs later and then 109 48 hrs later. OB wants another sonogram from a week from my last one. My first sonogram showed nothing but did see "free fluid" and thickened uternine lining.

My concern is that of an etopic pg that just wasn't able to be visulized on the first sonogram. I am curious what would make my levels climb every so slightly   - except that this may be an etopic.

Any insight is appreicated.

[slightlycosmo]

I would definitely want to have an ectopic ruled out.  Hopefully that's not the case, but I would have the same concern.  It could be that the pregnancy has implanted but isn't growing properly; that might cause very slow hcg levels.  However, I would definitely want to be certain I wasn't dealing with something outside the uterus.