Sorry I didn't notice your post before Pomtzu. It would sound like he's playing mind games with you but we all now they won't do that...will they?...
I think you're right about getting him checked out. I'm wondering if this could be tied in with the heart murmur? Did the vet say it was causing him any kind of discomfort, it doesn't usually but who knows with dogs.
Yes, I think one dose of aspirin could make a difference. My dog had a bit of a strenuous time Sunday and I noticed he was in a lot of pain after we got home so I decided to give him a dose of Metacam and in an hour he was his old self again. He hasn't needed any since but he just goes for daily walks as usual . Let me know how he's doing when you get some news from your vet, sure hope it's nothing serious. Does his leg feel warm to the touch?