One of two things is happening. 1, you are injecting into a vein EVERY TIME, which seems very unlikely or 2, you have developed a hematoma, a collection of blood outside of the blood vessels, pooling in the muscle. I have had this happen after using a site for a long time, even when I have the location on a one time per week basis. Injection technique and gear itself can cause trama to the muscle causing this to happen. I would take this as a sign to give the site a rest for awhile. I would stop injecting that glute for a month. After that time, I would do some sessions of deep tissue massage to help break up any scar tissue that may have built up. Even when you use a site once a wk, you can still create scar tissue that will effect the health of the muscle. Deep tissue work will help to break this scar tissue up and increase blood flow, which will help to carry away waste. If you are not using your quads, you might want to open them up, as well as your delts. Even when I am doing a minimal number of shots per wk, I still like to rotate though about 6-8 seperate sites.