should i go back to the emergency room? pregnancy? stress?

well last month and about 2 weeks ago i had unprotected sex but i was on birth control and he did not ejaculate inside of me just maybe pre ejaculation.
i was on the pill for a month when this happened.
i went 2 the doctor took a preg test came back negative.
took 2 1st response test came back negative and i recieved a full period at the end of september.
now i am experiencing bad stomach pains all over especially on my left side..i went 2 the er last night they took a urine sample,x ray & 5 tubes of blood.
the lady said my preg test came back negative.
if i was pregnant would it have been seen in the x ray or in my blood test even if they werent taking a pregnancy test?
ectopic pregnancy?
my diagnosis from the er was really bad constipation but i didnt know it caused really bad stomach pains…
any advice?
thanx