RH negative and bleeding?

I'm 24 weeks and 6 day's today and when I pee it takes a lot of effort and toward the end of my stream I find some light pink and brown drips in the toilet. I've been keeping a pantie liner on to watch it, but it never comes out unless I pee. I'm RH O- and I'm worried about my antibodies and my babies are mixing and that's whats causing this light bleeding. I'm calling my OBGYN first thing tomorrow morning, but I was wondering if anyone else has experienced something similar? Another RH negative mother out there with some advice?