Hello, Ok My Problem Is That I Was Expecting For

Hello,
ok my problem is that I was expecting for my period to come on March 10, however my period didn't show. On March 17th I started to see very light pink spotting that went on until about March 19, so March 19th I noticed just a little more than spotting blood but however it was not a normal flow so on March 21 I decided to go the ER because as I wiped myself a medium size blood clot came out of me but still just light bleeding on and off. The hospital did a sonogram but didn't see anything and when they did a blood pregnancy test my hcg level was at 16, they really couldn't tell me anything but to follow up with my obgyn! A few days after my ER visit my light bleeding stopped completely! I've always had a very low hcg level and it normally takes awhile for me to confirm that I'm pregnant anyway! Here it is now April 21 and still no sign of my cycle and should have come on about a week ago! Could the 16 hcg level mean that I'm pregnant but I was just to early for the baby to be visible on the ultrasound!