I agree with Lea. There will be nothing to worry about.
I had a week of bleeding at 16-17 weeks, they gave me a scan and said that it was because my placenta was low-lying, ie. covering my cervix. They said it would most probably move up and not to worry. This week I had my 20 week scan and my placenta had moved and all was fine.
If you do have a low placenta they will keep scanning you and according to my hospital they don't class it as a problem unless you reach 35 weeks and it is still low.