If you have a bovine valve, there's a risk that you'll develop spots, like the cow whose valve you're using. (I know, this is a terrible joke).
I can't imagine that those spots that you're getting are at all related to warfarin. it sounds like hives, or an allergic reaction to something in your environment, or, perhaps, insect bites. (Actually, with spring coming on, it sounds a lot like some kind of insect). If I had these sores, I would strongly consider seeing a doctor about them. (And if I thought I was having an allergic reaction, I may take a generic benadryl just to see if this anti-allergy medication helps ---OTOH if you have access to health care, which I DON'T, I suggest that you have a doctor check it out.).
I've been taking warfarin for 22+ years, and I've been on this forum for quite a while, and I don't recall others complaining of this symptom.
Good luck -- I'm hoping that this is a minor thing that can be easily cleared up. (And, again, forgive me for that awful joke at the beginning of this reply).