I'm surprised they have you eating at all this soon after post op. What phase are you on? I agree with Diane that you should call your doctor.
A few other suggestions are make sure you are taking small sips. If you are eating, everything should be chewed to a puree consistency. And eaten slowly, slowly, slowly!
Your stomach is basically a wound right now that has to do its job regardless of the injury it has sustained. Imagine having your thumb removed and washing dishes while the sutures are still in .. With no gloves.
You're not going to feel normal eating for quite some time. However, if you cannot keep anything down, call your doctor.
Good luck and feel better soon.