I have been using the MM CGM system almost continuously for the past 2-3 years and have had little problem with bruising. Last week I put in a sensor and when I activated it (I usually put in a new sensor at night and activate it in the morning when I get up - seems to be more accurate from the beginning that way). Anyway - when I activated it the reading was only about 6.75 & it stayed that way. I hadn't really looked at my sensor site since I put it in but after a few hours of continuous 6.7 readings I decided the sensor was bad (it was hurting a little also but not too much). When I looked at the site I had bruising about 1 inch by 3 in on 3 sides of my sensor. It had not even bled when I inserted it. Almost a week later it still looks like someone punches me in the stomach. I am curious if anyone else has ever had such serious bruising. It only hurts if I press on it - so I am not really worried - just curious - I must have gone through some blood vessels to cause the problem - but, like I said, there was no bleeding into the sensor - odd.
Sounds like a blood vessel to me, the bruising would just be blood under the skin. Probably healed quickly which, in my experience of having my CGM cause bleeding around the insertion site, means it won't change as the blood it is using to check has coagulated and isn't flowing back into your blood stream.
At least that is my impression of what happens. I'm no expert or a doctor though. :-)