Sounds to me like a missed shot, but there is no way for anyone here to know for sure. Keep an eye on it, if it gets worse - such as getting bigger, change in color, pain or anything like that than a doctor might be a good idea. Impurities in the dope, if it is indeed a missed shot, can possibly cause an abcess, and you do NOT want to go through that. Keep an eye on it.
I shot black tar heroin for many years, and whenever I missed a shot, i would gently massage the lump and it would slowly dissipate.