Clots are the Natural Healing Process of the body, but be careful and listen to your
body,
I've had this happen a few times myself and due to my vocation I can state a few things for you. Blood when moved into a receptacle that's not same as it's natural environment such as a rig can start coagulating (clotting) quite quickly as it doesn't have the same environment that usually stops this process. That's why bloods must get to labs asap and once there are put into machines that keeps it separated for testing. The liquid potion: plasma, and the cellular portion; red blood cells and white blood cells. Last, the smallest are the platelets. The smallest of blood's three cellular components are the platelets, named for their resemblance to tiny plates. Their main function was discovered when doctors observed that people with low platelet counts were especially vulnerable to bleeding. It is now well known that platelets are vital to blood clotting. When they touch the roughened surface of a torn blood vessel, they burst apart, releasing chemicals that set off a reaction in the blood leaking out. The result is that they convert one of the plasma's proteins, fibrinogen, into a network of fibers that trap RBCs - thereby forming a clot which seals the leak.
So not uncommon at all to see these frequently in rigs.. As many other threads mention, use a new one if you're going to reuse the product you're injecting. YES! If you can always use a new one as they're sharp, cause less tears to the vein thereby reducing the production of platelets. The more damage the more clots hence hard thickening veins as they trying their dam hardest to heal you. I buy them in bulk 100 for $25 AUD but not everyone has access to medical supplies. Impersonally use mine 2-3 times where I will rise thoroughly with boiled water, just don't try to clean the tip as nothing in our environment is clean (sterile enough) I've lost a vein due to the removal of a subcutaneous cyst which was full of a bad hit that became infected because I rushed the process, DONT EVER RUSH, no high is worth 2 days in hospital with surgeons, drs and nurses all knowing how you got there. Learn to do it yourself too, which I'm glad you are, talking risks like IV drug use means you have to be responsible for you, NO ONE in our science culture will ever care about you as much as you do deep down no matter what we are escaping, enjoying or whatever verb fits here for YOU. Stay safe....