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
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