Unless the pill was physically vomited or coughed out before being absorbed, then this gagging that you mention should have no impact on the effectiveness of the pill.
You mention that the pain is very slight. Pain in the pelvis can be due to problem with ovaries, tubes or uterus however it can also be related to the bladder/urinary tract or even the bowels.
If you are worried then by all means use a condom for peace of mind. Spermicide may irrigate the vaginal cavity leading to increased risk of infections so there is no need to use additional spermicide if you have good technique with the condom.
In order to determine what is causing the pain then you may need to see a doctor for full investigation and examination. However pain at scale 1 out of 10 is very mild and at the moment you can try some tylenol or ibuprofen to see if that helps.
Any fever, projectile vomiting or pain over 7/10 then go to ER in that event.