In the sixth day of shiva nekiim, I saw a tiny amount of blood on the bedikah. I did 2 more bedikahs after, and they were completely clean. It was late friday afternoon so could not go check it out internally. Should 7 days be started over, or is there another way?
The seven days need to be started over, unless it can be shown that the blood emerged from a cut of graze, rather than from the uterus. This does not appear likely, because the other bedikos were clean, indicating that there is no wound. Assuming this is the case, the seven days would begin again, the other bedikos you performed being the hefsek taharah.
I know this is not “good news,” but don’t forget that the entire point of the bedikos is to “look for blood,” and if you actually found it, there is nothing to do but begin again.
Having said this, it is unsual to find blood on the sixth day, and you should ensure that the red coloring was not there before/a red thread that got onto the cloth (this sometimes happens), or a hidden cut or graze. The only way to ascertain the last cause is by doing an internal check, and as noted, it does not appear likely in your case.
Best wishes and good luck!