I have exactly the same thing at the moment and I've been doing some research into the possible causes of the pain.
I also find that lying on my back the pain has been really bad and everything else you described sounds like exactly what I have. How does sneezing feel? For me, pretty painful.
It's not a normal hernia but it could be a sports hernia which still requires surgery to correct. I say could be because it's really not that common but the sport you injured yourself doing is the highest cause of it. I believe it's called Gilmore's Groin.
My injury came on during high mileage marathon training. It started off as a dull ache, then went into the painful stage that it sounds like you are at and now it's moved to a more general lower abdominal discomfort. It does feel like it's getting better though so at the moment I'm playing a waiting game, avoiding pain where possible and seeing what happens. I'm nursing it just like I've nursed other injuries before. I also don't have the option of taking weeks and weeks off as I'm building up the mileage for an ultramrathon in December. I'm going on the theory that not many runners get a sports hernia and it's most likely just a chronic sprain caused by high mileage training. It certainly does'nt affect my performance, I ran a race on Sunday and didn't feel a thing. Football though is a different thing though as it requires very different movements to running.
For the record though, I'm not anti seeing a GP or a specialist but I want to see whether it's improving or getting worse. Should it suddenly change I would cease running and get medical advice.