What causes blister with itchy pain at the tip of penis?

Hello doctor,

5 or 6 days ago, I slept all through the night wearing only a boxer. The next day, I noticed a slight itchy pain closely at the tip of head of my penis. The next day, I saw a dotted blister at same spot where I noticed the slight itchy pain. At first, I thought it was the work of an electric ant, because presently at the area where I live, there are electric ants. So, I decided to use antibiotics. But the blister is now spreading, getting to my penis mouth hole.

After a week I went for a medical checkup and when the result came out, I was tested negative for some STDs. The doctor told me it is an infection from bad toilet and water. He prescribed Aciclovir tablet 400 mg, Aciclovir cream and Piriton tablets. Now I am almost done with the drugs, yet injuries have not healed. Two days ago from now another small red blister came out. I am scared and worried. What could this be? STD? If not, what could it be? No painful urination, no serious itching scratch, no serious pain. Age: 21, gender: male.