What is the matter if I had a little bleeding on birth control pills and I'm not pregnant?

I've been on Loestrin Fe24 for 2 years now, and my periods have been regular. I take my pills religiously (the regular pill, not the mini pill) every day and I have not missed one. My periods have been like clockwork for the last two years, and I suddenly missed one this last weekend. I had a little bleeding that appeared to be the start of a period, but then it all of a sudden stopped. I took two home pregnancy tests on different days after my missed period/slight bleeding and they were both negative. I am ually active, but as said before, I take my pills every day at the same time and I am not pregnant. What could be the reason that the bleeding for my scheduled period was so light and short?