Yep, been there! When my stomach was empty in the morning I'd sometimes throw up a yellow bitter fluid. It's mostly bile and just means there's nothing left in your tummy. It can happen around 17 weeks as hormones relax everything. Try to keep a little something in your stomach by eating small snacks often, and sip water or ginger tea. Of course if you're vomiting blood or can't keep anything down, give your midwife a call, but occasional yellow bile is super common.