Rather than STD I was thinking in terms of sensitivity to the material of condom or spermicide as this can cause itching or burning with intercourse. At least we can rule that out.
I would suggest a re-swab for two reasons. Firstly to verify that the bacteria is susceptible to ampicillin. Resistance to antibiotics is becoming more common so you may need an alternative antibiotic. And the other reason is to see whether the Strep may have cleared up but ampicillin very often causes a secondary fungal infection. This would also cause pain and burning, although discharge can occur too.
Some things to rule out would include allergy to perfumed toiletries or laundry detergent, also excessive douching leading to pH changes of the vaginal canal.
Finally if there is no evidence of any bacterial involvement then oestrogen deficiency may be a factor. Certainly its worth discussing with your doctor as a final alternative. But the first step is another swab.
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