This could go one of two ways
Ok on the good (and more likely) side it could be just a little urine or gas in the lower bowel causing that feeling I've seen that..(then they pee on you)
Now for the scary side you could have given the injection wrong (i.e. to steep an angle or to deep) and done damage to the intestine and the swelling is due to this trauma..
But I'm more inclined to think its the first