Well you aren't a crappy person at all, you're Bipolar...there's a difference
My only thought here is you should absolutely tell your doctor because the effects marijuana can have on meds isn't clear nor is it clear what exact effects it can have on an unborn child. Enough of my this is what you need to do.
Let me tell you what I didn't do....I didn't stop drinking or smoking pot and the health risks were amazing. Although my child ended up a sweet, beautiful, intelligent little girl; my life was almost taken from me because of the choices I made while pregnant. I don't think you are an awful person at all for it, how about considering just stopping until your little one is born? Then you are free to decide to go back to it. If you have a problem with coming off then definately seek medical help with it. Thank you for your honesty. Trust me when I say you have helped someone today.