hi jen, the reason I asked how it was diagnosed is that I had a very swollen anterior muscle a couple of months ago - it was very, very puffy and I was told not to do anything with it until the puffiness settled. It sounds very similar to yours and I rested a week in total from running and after that could walk on it without pain and then run without pain - I'm glad to say that it hasn't come back (just have a million other niggles and problems instead of it now!)
I was told that compartment syndrome was something very specific which would return and was actually better diagnosed with some sort of pressure test that measures how much pressure the muscle is putting on the surrounding compartment (or something like that). However, I think that's if it's something that keeps returning, rather than an 'acute attack' like I had.
I found that anti-inflammatories helped as well, don't know if you are taking any or not.
Good luck, hope it settles down.