This exact thing has happened to me twice. I was on M1T each time it happened, and yes, it is a tear. The first time was in my right tricep on the inside of the arm near the elbow, and the second was just a couple of weeks ago on the left tricep near the armpit. The second one was bad enough to bruise all the way from that spot to just above the left lat. My wife saw it and got all scared. It is almost gone now--I just posted a couple of new pics in the member section taken last week, and it shows as a dark area under the arm that I'm flexing in the second pic at the end of the thread.
I'm pretty sure I overdid it on the second bruise. The first one seemed to show up out of nowhere. It is possible that M1T pumps you so hard that the muscle is unable to stretch out after a certain point and then tears if you continue. It is hard to get the tris stretched out well so I can see why it would be the one to tear first.