im not sure myself. and i had a miscarriage, though i decided to go for a d&c instead of wait (it was around Christmas time. i didnt want Christmas to be a mess because i started to shed or something...)
i have heard that there is sometimes implantation spotting. but its usually nothing big, but i havent had this myself.
what you can do is just pray, and try to relax. too much stress can make things harder... i know its a terrible time for you, but just have peace that God has it under control.
one thing that helped me (after my miscarriage) and was something i still would have liked to have heard when i had it was that the simple fact that you have a miscarriage means that your body is pretty much ready to be pregnant... that you are able to get pregnant, and your body is gearing up for it. kinda like a preparation. im very sorry if this turns out to be the miscarriage, but thrilled for you if its not! :D just remember that sometimes, God has other plans. and i have a small inkling of belief that every pregnancy, no matter how long its alive, is a child, and i believe that we will meet them in heaven someday.
however, it certainly feels much better to us if we meet them now, when we were supposed to have. miscarriage is a frustrating, hurtful experience.
however, i thought that hcg was the thing you get when you are pregnant, my levels dropped when they started thinking miscarriage. soooo.... with the lightness of your spotting and the levels going up.... i would probably be more likely to think that its a viable pregnancy. :D congrats!
just... walk on eggshells i guess. tread that fine line between calm and sensible worry. drink lots of water, take a vitamin, eat very healthy foods - fruits and veggies, whole grains, legumes. LOL. walk, breathe, relax. good luck and God be with you.