I once went to a workshop run by a well-known acapella group, and when someone asked how they kept in pitch, they looked alarmed and said, "Whatever you do, don't test our recordings by pitching the first note against the last!"

I can usually keep in pitch, but if I wander at all it's in an upward direction. I think this is perhaps a sort of over-compensation against any tendency to flatness, because for some reason flatness sounds worse and more noticeable. Big choirs are more likely to go flat (I blame the tenors!) while solo choirboys tend to go sharp. So you're not alone.

Now that you're aware what's happening, you may well find that you are correcting it and keeping closer to the pitch you started in. If you're singing unaccompanied, almost no one will notice a slight upward drift anyway.

If you're recording acapella and want to keep it in pitch, one trick is to wear headphones or an earpiece plugged to a keyboard, and stick one keyboard note down (probably the tonic or keynote of your song) so that you hear it as a drone in your head. But as I said, my guess is you'll manage to stay true to pitch now, or close enough.

Marje