Just wanted to let you know that I had exactly this too when I was pregnant - I think the first time was around your stage and then I had it again about 16 weeks.
It was just on my knickers though rather than when I wiped, but I'm guessing probably about the same sort of amount.
I was told it's very common to get old blood dislodged when the baby starts to wriggle about a little bit and as long as it's not bright red and there is no cramping, not to worry. I phoned the maternity unit both times it happened, and neither time did they worry or want to see me that night, unless it turned bright red or had cramping.
I did go up to the maternity unit next day both times to hear the heartbeat for reassurance.