Concerned at 4 weeks

I am 4 weeks pregnant, and I've been bleeding light to moderate amounts for the past week.  Last night I passed a small amount of tissue, and it was fairly reddish color.  I've taken 4 tests over the past week, and they've gone from very faint positive on the first day, to very solid positive the past 2 days.  Although the tests would indicate that I'm heading in the right direction, the bleeding and tissue passage is very scary!  Is it possible that my pregnancy is ok?  The fact that I have very little cramping gives me some relief, but I'm still freaking out.  I have my first appt with my OB later today.