Don't do gardening.
The only thing I can think of is there are a lot of sheep and cattle in the field right next to the house. Also, there is a bit of a green lane, bordered by trees, at the back of the house.
So perhaps either of those could be it.
A blister did start developing on it last might. It got ****ed with a sterilised needle and covered with savlon and a plaster. The swelling reduced pretty much straight away, it did come up a bit this morning, but my ring isn't too tight, phew
Just thought, there is a car port which we use quite a bit, so something could be lurking there.