What did you have on the bagel. I get this reaction from all types of dairy within 5 min ...usually almost instantaneously. I have a dairy intolerance as none of the Lactaid products or pills ever did a thing. I go the other way though..not D but C and I can totally relate to feeling spacey and tired. I lose any concentrating skills whatsoever...like I am in a fog and it is sometimes accompanied by a severe headache and sore swollen glands.