What Causes Red Colored Rashes On The Arms?

I Have a red rash on my arms (inside),hands,& legs. Almost like blister,some wit heads & sime simply large red spots. I ahve no feaver,nor itching. I went to my PCP a week ago & she thought I had contracted some allergy from a bush or a plant because I work in the yard(wwwods) during the weekend. I have never had anything like htis & it is not clearing up any. I am taking cortisone wtih acid in it,but it really ahs not been effective. All it does is fade the spots,but then continue throughout my arms,legs & even foot. My BP is good & it is almsot liek I ate something I am allergic to. Someboyd suggest it could be from arthritis & somebody else said it coluld be shingles. To my knowledge I wasn t bitten by an insect,as least I didn t feel any bites. I am in my 70 s & ahve had recent backaches(I think arthritis whic I haqven t addressed wti hteh Dr.). Any suggestions?