hard lump to the right of my left ribs

For the past 7 or 8 months the bottom 3 ribs on the left side of my rib cage have been sticking out more than normal. I didn't really worry about it though, I figured it was normal for bones to grow a little differently sometimes. But a couple of months back, after poking around my ribs feeling just how different they were from the ones on the right side, I realized that I could feel something strange. I'll try and describe it.

It goes in the opposite direction of my ribs (up and down my body, in other words). It's a few inches long, probably between 2 and 3. It's rock solid. It only shows up if I've been lying on the left side of my body for about 10 minutes, otherwise you can't even feel it. When I press on it, it recedes into my body, and when I take my hand away it pops back up again. Sort of like pressing on a jar lid that pops in and out. If I push my finger down the side of it and press it against my ribs, it doesn't give at all, as previously stated it's rock solid. I've tried getting my fingers down the left side of it to get a better feel of it, but it's too close to my ribs for me to do so, and I end up just pushing it back.

I went to see a doctor about it, but he refused to let me lie on my left side for 10 minutes for it to show up. He's a busy man, as there's only one doctor in town, so it's understandable. I described it to him, and he simply said it was my spleen after dismissing everything I had to say. I find it hard to believe that it's my spleen, as I can't see why an organ would be rock solid and just come and go like that. And it seems like more than a coincidence that it's near ribs that are sticking out more than normal.

I'd like to get a second opinion, but I'd have to drive for about an hour to find another doctor, and I doubt I'll be able to find one that lets me just lie there for 10 minutes so the thing can show up anyway. Thus, it's very difficult for me to convince myself to go out and see another doctor.

Does anyone here have any idea what it could be? It doesn't cause me any physical discomfort, but it's somewhat concerning.