The test are very accurate. You probably are just having a late period or skipping a period. If you don't have a period in the next cycle then take another pregnancy test. If you are not pregnant and still not having your period then you need to see your doctor.
You need to go and get a pregnancy test. The withdrawal method of birth control is not effective. The anaesthesia used for the surgery or even the surgery probably would NOT cause your period to be late. If you are regular then you are most likely pregnant.