If I remember correctly when I took mine out after leaving it in for an extended time (more then the 3 weeks) I too started like 24 hours later. I think what you are going through is normal.
As far as the stomach issue, sounds completely unrelated to me. I mean traveling and different foods can REALLY take a tole on the stomach and digestive system. Being that you had your ring in the whole time I would not worry about being pregnant.
Hope that helps, and hope you are feeling better
