My string loop broke yesterday. So I used the little mark on the string left by where the nock on the arrow had been making contact as a reference and tied another string loop on. For some reason my arrows are now hitting low. What did I do wrong ?

Move the loop down a serving strand and see if it corrects. You may have it a strand too high up; doesn't take much.

Thanks for the help !

It's amazing what moving it 1/16" can do