Could bleeding that last for 5 days, starting out like a light period and then turning into spotting, be implantation or breakthrough bleeding?

Take a test: Home pregnancy tests are very effective for determining if you might be pregnant. It's hard to answer your questions specifically as you didn't state where in your cycle you were. At 16, if you're not pregnant, a reliable birth control method would be a great idea. Pills and condoms are great for preventing pregnancy and protecting you from stds.