Round water blister on right arm

[Nickers67]

One lady who lives in a house here in Australia Queensland Brisbane she is 72 a diabetic she gets the blister that everyone is talking about,hers is round blisters on the right arm and she only gets it this time every year, I know it's not dermatitis because I suffer from dermatitis. She uses an antiseptic wash she doesn't go out in the sun she eats 6 meals a day to control her diabetes she doesn't drink any more or smoke. Hers gets itchy then a few days pass and it gets red and sore the dr doesn't do anything about it. I have googled it but there are millions of things about this. If anyone has had test please share it's just so weird. I know with my dermatitis the dr tested my skin and found that grass sap made my skin break out, but since I have stopped eating gluten it's gone away. Some people can't change there diet specially if they are diabetic . Thanks so much for taking the time to read this