Pregnancy question please answer?

i went to the dr. for stomach pains and they had me do a urine test to check for appendicitis. they also tested for pregnancy and said i am not. They did a cat scan to check my appendix and said there was nothing wrong with it. They would have seen if i was pregnant then too, right? For some reason, im still concerned about if i could be or not. i havent done anything with a guy since way before the dr's visit. am i over reacting? or is there a chance that i could be pregnant-- even when the dr. says that im not? i had just had a normal period a week before i went to the dr. my period had just ended a day before i went. it was a completely normal period for me... 7 days 1st day cramping... the whole shebang. and this is going to sound gross, but the reason for my stomach problem was that i was consuming way too much fiber without drinking an adequate amount of water. so, for lack of better words, i was err clogged.