I assume you are using regular needles to draw up. The chunks of rubber are from the needle taking chunks out of the rubber topper and depositing them inside the bottle. This is known as coring. There are even smaller pieces that are almost impossible to see that can be drawn up and then injected into the muscles where they will remain indefinitely. To prevent this, draw up with a filter needle, then switch to your injecting needle to inject. This also is less painful because if you pin with the same needle you drew up with, the needle is dulled from just the rubber topper.