I had an ectopic pregnancy last May. I was pregnant for two months without knowing it. I had two light periods during that time. Then at the end of the second period, I got some bright red blood and a bit of cramping on one side. I thought it was just something odd about my period and ignored it, but it didn't go away. After a few days, I took a pregnancy test just to rule that out and got a very faint line. I knew that meant something strange since I had to be at least two months pregnant (based on last possible conception opportunity!). I went to the hospital's walk-in clinic and they got me an ultrasound, where they saw a mass on my ovary. They put me under a general and did surgery the same day to remove it, and my OB confirmed later that it was an ectopic pregnancy. (They thought there was a chance it was just a cyst.) I had had no symptoms until the rupture (what caused the bleeding), which would have tipped me off if I'd known I was pregnant, since I should have had pregnancy symptoms and I didn't.
From what my doctor said, low hcg levels are a strong indication of something being wrong, like an ectopic or a miscarriage in progress. If I were you, I'd check a test strip now if you have one and see how dark the line is, just to get a better idea what's going on immediately. Then get an appointment for an ultrasound as soon as possible, to find out for sure.
I hope you're just having some cramps and nothing is wrong. Good luck!