Shoulder pain?

[Veelah]

My dog has been limping for the past few days.  I noticed it before while he was outside, but I thought he was just holding his paw up to keep it out of the cold snow.  Today I noticed him keeping his weight off that leg.  It's the front left leg.  At first I thought it was just his paw, but it wasn't sensitive to the touch.  Then later, he was lying next to me and I sort of leaned over him and he squealed in pain - that's when I realized it's actually his whole shoulder, and maybe some of the upper leg, I'm not sure.

Three days ago I was clipping his nails, and as usual he was squirming like a maniac.  He's a big lab / great dane cross, so he's extremely strong.  He and I were fighting over his paw, and he squealed at one point so I stopped for the day.  I didn't think anything of it at the time but could I have dislocated his shoulder?

I just looked up what sort of pain relief I could give him before we go see a vet and so I gave him a crushed Aspirin tablet.  He was lethargic all afternoon - even the word "outside" didn't inspire any excitement in him.  I'm really worried.  Is there any possibility that a dislocated shoulder will heal on its own, or is vet care absolutely necessary?