I am curious to which antibiotic was used in the mix..
I am sure the vitamin was a B12 mix, which is typically a dark brown/orangish color, which can 'stain' the skin on thin skinned animals like leos.
In combination with antibiotics, which are dehydrating, it can sometimes make the dark coloration more prominant, due to the dehydration effect in the injection area.
Antibiotics alone, especially baytril, on the other hand, can cause slight tissue discomfort in the area and sometimes will leave a slight harmless abcess under the skin. The majority of the time, the abcess will go away.
Another point to consider is the gauge of the needle used.
Smaller is better on small animals, as long as the medicine can easily flow out of the syringe. If not, then the meds can most likely be thinned by using an injectible saline to dilute it.
Think of the needle size in comparison to our size.. if we were to get an injection from an apportianately sized needle, it would be like getting a shot with a 1/2 inch pipe.
Yes, we would bruise AND possibly get some damage to the skin and tissues just from penetration alone.