Spotting during the first trimester!!! But HCG still raising

So I went to the ER on the 7th because I wasn't feeling well they checked my HCG levels I was at 344 they did a pelvic ultrasound couldn't see nothing and did a pelvic my cervix was closed went back today because I started spotting not even enough too make me even have to change a panty liner just occasionally when I pee and wipe I see blood did my HCG levels they are at 789 today so my question would be do you HCG levels start to drop as soon as you start to miscarry or do they continue to raise somebody please tell me you know this answer doctors or unsure what to think since they can't see nothing on my ultrasound at this point? Oh one more thing on the 7th they saw a cyst that was resolving and said I had free fluid in my pelvic area could that be causing the spotting? Have anybody been trough anything like this