9 week old can't poop

I know It's normal for newborns to go a few days without pooping but this is the longest my girl has gone without going. Her last bm was Saturday night...it's now going on Wednesday and I know she is trying to go but can't. She's had lots of really smelly farts and has been abruptly waking up from naps screaming & crying like she's in pain or discomfort. She is pushing/straining, kicking her legs, & arching her back. I know it's not just gas at this point. She needs to go poop but can't. HELP!