Pain on my tongue, whats wrong

Saturday i went to the doctor and found out that i have a bladder infection. so i took the big white horse pills that they give me, i have to take the pills twice a day for three days. monday was the last day for the pills and tuesday is when i notice that i have a blister or something on my tongue. when i try to eat it hurts on my left side (thats where the blister is located) i can barely talk because it hurts when i try to do that. please tell me whats wrong