I don't think a tap on the bumper that didn't damage either car would cause a miscarriage; it's probably just coincidence that you started spotting.
A little advice from someone who has been there? It sounds to me like you are constantly thinking about the next baby, and it is ruling your mind. I understand, because I had two miscarriages between our two boys, and I was totally where you are. Every month I thought I might be pregnant, and every period was a crushing blow. We did eventually have another son (5 years between, 3 years of trying) with no complications. I drove myself crazy for those three years. I'm a control freak, and it really got to me that this was out of my hands.
The best advice I can give you is to enjoy the life you have. Yes, work with a fertility specialist, but try not to let it take over your life. You have a son who needs his whole mommy, and these years are precious with him. Love him and enjoy him, and if #2 is meant, it will happen. Unfortunately you can't control when. What you CAN do is see a fertility specialist, and pray--and not just for another baby; pray also that you can accept and enjoy your current one-child family. It's not what you had planned, but it's still awesome.