| Yes, you should seek another doctor's opinion. A used razor can harbor staph or e-coli bacteria germs from skin that is stuck on the blade. When you cut yourself, the germs entered your skin thus the infection. The doctor needs to do a test to determine what type of infection you have and then work with you to find an anti-biotic that works for you. Don't let it get any worse. Call a doctor immediately. If it is a staph infection, it will kill the tissue and eat away at the skin.
As far as a STD, you said you were tested. Was your boyfriend tested too? If both of you tested negative and have been faithful to each other, the chances of it being a STD in slim to none.
Hope this helps. | 
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