What are the chances?

I've been hiding out and sorta lurking around here lately, but I think I did
a slight uh-oh with my dates.

Day 1 of my last period was March 7. For some reason, I was thinking it was
March 12, although I don't know why I had that date stuck in my head.
Rather than double checking on my calendar, I had that day set in my mind,
yet what was written on my calendar said otherwise. So, this week, I was
thinking 5 days ahead of what I actually had.

Dates of intercourse lately (last week) are as follows...
March 17, March 18, March 20.
It's only March 21 today, but what are the chances for conception? Good or
bad?
My cycles always seem to be the "average" of 28 days exactly. Each and
every month, and always bang on.
No form of birth control, but we haven't been trying or anything like that.
I've been keeping dates on my calendar since DD2 was born (and she's 17
months in a few days)
When we've tried, 'trying' lasted only a month, so we've never had any
fertility or conception issues. It was more decide to try and that month
has always been a go (4 times so far and that was that) but when we've
decided to not try, we've never had any surprises at all.
Obviously a test is the only thing that would say for sure and obviously
it's far too early for that, but when I fished around google a little,
everything was pointing to a fairly high chance of conception based on these
dates.
Just wondering if I'm getting all worked up for nothing or possibly not.