Thanks for query,
As your blood test negative for pregnancy there is no chance of pregnancy not even ectopic one.
Your period also came suggest that you are not pregnant.
the reason of your right abdominal pain
radiating to back may be due to your delayed period & too have other possibility like appendicitis
,ureteric colic or bowel disturbances.
Just watch for other symptoms then pain like loose motion
,burning micturation vomitting etc if this happen you should go to urgent care.
A mild anti spasmodic like dicyclomine
will serve your purpose.