I Have A An 11lb Min Pin That I Rescued

I have a an 11lb min pin that I rescued from shelter about 2 mos ago. He has been very healthy, lively and good appetite. I only feed him blue mt small dog twice a day( adequate portion) this morning he woke up and went out normally to pee and came back in house and went back to sleep. When he went to get up next time about an hour later he could not walk on front left leg at all( no support whatsoever). Only one of the front legs though. He managed to stand to eat his breakfast but then basically just collapsed again. We gently touched it, no fracture because he shows no sign of pain. The joint did seem a little swollen. Now it is 5 hrs later and he is really stiff on the one left front leg ??? we did go boating with friends yesterday and we think someone could have given him a nut or piece of a cooked shrimp . Could he be having an allergic reaction many hours later ??