I've had lots of problems trying to conceive. I've had 5 miscarriages. BUT I've also had 3 healthy children. One just last week for that matter.
Anyways based upon what you are saying it could be a miscarriage or it could just be normal bleeding. Pregnancy tests can usually detect HCG at levels of 25 all the way to 100. So as you'd guess it depends on the brand you have, how concentrated your urine is when you take it, lots of thing as to whether you get a positive or negative.
This site is great for pregnancy test information, how accurate they are.. etc. peeonastick.com
Plus 50% of bleeding I've been told time and time again can still mean a healthy pregnancy. Although usually most think that red blood means miscarriage that isn't always the case either. With the baby I just had... I had bleeding from week 6 to week 20 something. Every time I'd go in there he'd be with a heartbeat. So it doesn't mean the end all.
Anyways at this point you need to go in Wednesday and get a quantitive HCG done. I'd go back in 2 or 3 days and get another one done. That way you will know if it's increasing or falling. If it's falling most of the time that is a good indicator of a miscarriage. If it's doubling so far so good. At 6 weeks pregnant they can do an ultrasound and usually see something and with me I've always gotten a heartbeat on my sucessful ones. If you see a heartbeat at this point it's a great sign that things will be great.
But at this point it could go either way. You have signs of miscarriage but that doens't necessarily mean you'll have one. So I think you should just try and stay calm and go to your appointment on Wednesday and get the blood work done. Hopefully everything will come back ok. I know how hard it is to sit here and wait but unfortunately that is all that is going to tell you at this point. I wish you the best of luck! I hope your HCG is doubling.