Hi. I Got Bitten Whilst Riding Out On My Horse

Hi. I got bitten whilst riding out on my horse two days ago. On the back of my leg just above my knee.
I've got several blisters that have come up.
But one has come up ontop of a surgery scar I have. Just a little worried. I put antihistamine cream on it and it's made the blisters worse.
They've been weeping all day. The red swelling has got larger. My knee is swollen too