Chances that I could have an STD?

Well if he did have something that could be transfered via blood this would be the most sure way to contract it because you are putting it directly into your own blood stream. I'm assuming he has shot up numerous times with other people and if he shared with them as he did with you, you never know so I suggest you get tested. You can go somewhere like planned parenthood or a local clinic and get it done for little or no cost.