Looking at that my first thought would be a wood chip or something, but normally aspen shouldn't do that. I guess there's the possibility that you could have ended up with a stray wood chip in your aspen (I've found rocks in eco-earth before so idk if harder woodchips could get into aspen shavings).

So when you say you don't feel the lump at all, does that mean the lump isn't hard? Does it feel solid or fluid-filled?