Is the bump reddish and warmer than the surrounding skin? Do you feel sick in any way? Check out the signs of an abscess or other dangerous conditions and see if you have any of them. A "lump" can be caused by all sorts of things (many of them not serious), that's not a very specific description and doesn't give us much to go on other than the "better safe than sorry" answer. In my experience going to a doctor or clinic about stuff like this is way better than going to the ER of the hospital. I guess it depends on where you live, but in my experience at the ER you have to wait for a very long time just to be treated like shit by the ER staff who think as a drug user you deserve anything bad that happens to you. And if they think you are a "danger to yourself" because you use drugs illicitly they may just lock you up - that's what they did to a friend of mine who told the ER doctor she injected heroin. I'm not meaning to scare you out of getting something potentially serious checked out, but if you don't have any symptoms other than a lump it may be worth it to wait and go to GP or a clinic as opposed to the ER.
