sometimes if you inject too shallow some of it leaks back out into the subdermal layer and causes irritation
also if you move around the needle too much (shaky hands) it can tear up the muscle and cause irritation.
the hardness could also be the gear crashing, sometimes it causes a swollen lump and a bit of heat and redness
if it doesn't hurt to put pressure on it, and feels like a hard lump, it's probably scar tissue from injecting there too much, don't use that site for a while and massage it to break up the scar tissue. occasionally if it gets too bad you'll have a hole there once the scar tissue goes away. i had one on my delt and my calf from bad b12 a while ago but they seem to have filled in, mostly
also i've gotten lumps and redness before from using shit gear. sometimes a bit of fever too, but it might just be the "test flu".
more than likely, you'll be fine, just get plenty of rest and vitamins. however, if you get a bad fever or excruciating pain or if the lump doesn't start going away in a few days, i'd go see the doc.
