Diabetics always run a very high risk of infections due to uncontrolled blood sugar which helps the microorganisms to grow faster and defective immune system to fight the infection.
The clear fluid filled blister
may be due to and heat or pressure effect to that area due to friction.
But now it has become red and tender. This is a sign of inflammation and infection.
You should have your blood sugar under control.
Go for I.V antibiotic and good wound care.