That is an excellent question, and based upon the limited history available, I think that the best person to evaluate you and check your history and physical examination would be your primary care doctor
or ob/gyn. In terms of reasons that you are feeling nauseated after eating, there are are several causes for this and they can vary from quite a few things. One potential as you mentioned is that you may be pregnant, although if you are having regular menstrual cycles that is less likely. Another potential is that you may have an allergy to the foods you are eating, this can especially be seen with gluten or milk products. Another potential cause for nausea and vomiting
is acid reflux
that can become very severe and cause you to have these feelings of nausea. Sometimes this can also manifest itself as chest tightness or an acidic taste in the back of the mouth. Have you tried any medications that helps with this feeling, such as an antacid or PPI? Because there are so many potential causes for nausea after eating, it is very important that you are evaluated by a primary care doctor because they will be best able to figure out what the cause of this is.