I'm 17 years old. My boyfriend fingered five days ago and he accidentally scratched my vaginal walls. I had very little bleeding and then it stopped. The day after, I had mild cramps but eventually it stopped. And then this morning, when I woke up, I found a tissue-like discharge on my underwear and I had no idea what it is. I'm supposed to be having my period now. I'm having severe cramps too. Someone help me?

Menstrual flow is tissue. You are probably having your period with very little blood. Cramping can happen one month and not another, and also mid-month. You might start bleeding any moment now, and the cramping might go down.
Are you at all constipated? Women have all sorts of ways to deal with cramps. Warm baths and pelvic tilts are two, giving up caffeine in any form is another. Eat sparingly, avoid greasy heavy foods, drink lots of fluids, keep your bowels moving, do tilts any time you can, and keep your entire lower torso WARM.

That being said, there's a bit of a chance that any fingernail cuts let bacteria in under the skin. If you get a fever, have pain where the cuts are (not cramps), or an unusual discharge, see a clinic.

Sounds like you are about to get your period. Just wait it out a few days and see what happens.