Hi there Amanda.
I fear that you may be experiencing some symptoms of either vaginismus
or some pelvic inflammatory disease
It is very difficult online to comment sans clinically examining the patient.
Please visit a gynecologist on the ground nearest you for best results.
I understand you have been tested for STD's & are negative. It may be a simple bacterial/fungal UTI that are quite common in women.
It may be also related to the ovaries & cycles. Please get an abdominal ultrasound done & perhaps a transvaginal ultrasound
. Please do visit your gynecologist with these.
I could sugggest a treatment but that would be harsh sans all the tests & the clinical examination.
I do hope have given you enough information to make an informed deciscion & Godspeed..wish you good physical & sexual health in the long run. Cheers!
Dr Praveen Rodrigues MD
Vikram Hospital, Banglaore, India