Hi Miasma,
sounds like you've injected into the area on more than one occassion, and you have just discovered what happens. It is so very important to alternate injection sites, from one thigh to the other and in different areas, i know this is sometimes hard to remember, often helps to write it down. Give that area of thigh time to recover and steer clear of injecting exactly there for a while. Someone else will be along to give you the correct terminology, but don't worry this happens to most people every now and again. The lump will disappear.
Take care and don't worry,
Suzi x