Are you absolutely sure that your oral sex is connected with blisters inside your mouth?
And how long have you been dating your boyfriend? Do you completely trust him or you doubt that he has some sexual transmitted disease? It would be best to check with him if he had risky sexual intercourse before you and if he did than you will have to visit your doctor and especially if you had unprotected sex with him.
Your symptoms could also be a condition which is not connected with sexual transmitted disease. This condition is called mouth ulcers and it usually appears inside of the mouth and can be very painful. Fever is symptom which is usually accompanies mouth ulcers. If you haven’t resolved your problem by now consult your doctor as soon as possible.