Sorry, I must have deleted a line by error before posting. It's supposed to say gonorrhea and chlamydia are at the top of the list of possibilities; but there are many other possibilities, including pregnancy (especially with an impending miscarriage)... and the rest of my response is OK.
HHH, MD
Bleeding between periods has many possible causes. Every one of them needs an exam. In young, sexually active women, STDs--especially chlamydia and gonorrhea--are at or tcy (e.g., an impending miscarriage), fibroid tumors, and others. Injury during sex, including your partner's fingers, is not a likely cause.
Bottom line: Get examined soon, to include pelvic exam, pap smear, and tests for gonorrhea and chlamydia. It would be a good idea for your partner to get checked out at the same time. Do it in the next day or two; delay risks complications and a small chance of dangerously severe bleeding.
Good luck-- HHH, MD